ENH Support SDL-based packages and add pyxel. ### Description

This adds SDL-based package support to Pyodide and adds pyxel as a PoC. Most of the troubleshooting is done by @kitao and pyxel devs, a big thanks to them.

This is on top of #3505

1) I linked only js part of SDL libraries to the main module and linked the native parts to the side module. So the size increase of the main module is small enough (~0.1MB increase to the pyodide.asm.js).

Before (in #3505):

```
5145705 pyodide.asm.data
1204075 pyodide.asm.js
8862216 pyodide.asm.wasm
```

After:

```
5145705 pyodide.asm.data
1346524 pyodide.asm.js
8905514 pyodide.asm.wasm
```

2) Emscripten draws everything into `Module.canvas`, but the canvas attribute is not set automatically (which makes sense as we don't output any HTML), so it is the user's job to set it manually.

For now, I am thinking of just mentioning it in the FAQ entry, "you need to set pyodide._module.canvas = ...". But it would be probably nice to have an API to set canvas, as pyodide._module is a private attribute.

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

### Demo

https://ryanking13.github.io/pyodide-pyxel-test

(mostly borrowed from the pyxel repository: https://github.com/kitao/pyxel/tree/main/wasm)

export PYVERSION ?= 3.11.2
export PYODIDE_EMSCRIPTEN_VERSION ?= 3.1.32

export PLATFORM_TRIPLET=wasm32-emscripten
export SYSCONFIG_NAME=_sysconfigdata__emscripten_$(PLATFORM_TRIPLET)

# BASH_ENV tells bash to run pyodide_env.sh on startup, which sets various
# environment variables. The next line instructs make to use bash to run each
# command.
export BASH_ENV := $(PYODIDE_ROOT)/pyodide_env.sh
SHELL := /bin/bash

version_tuple := $(subst ., ,$(PYVERSION:v%=%))
export PYMAJOR=$(word 1,$(version_tuple))
export PYMINOR=$(word 2,$(version_tuple))
export PYMICRO=$(word 3,$(version_tuple))

export HOSTPYTHONROOT=$(shell python${PYMAJOR}.${PYMINOR} -c "import sys; print(sys.prefix)")
export HOSTPYTHON=$(HOSTPYTHONROOT)/bin/python$(PYMAJOR).$(PYMINOR)

export CPYTHONROOT=$(PYODIDE_ROOT)/cpython
export CPYTHONINSTALL=$(CPYTHONROOT)/installs/python-$(PYVERSION)
export CPYTHONLIB=$(CPYTHONINSTALL)/lib/python$(PYMAJOR).$(PYMINOR)
export SYSCONFIGDATA_DIR=$(CPYTHONINSTALL)/sysconfigdata/
export CPYTHONBUILD=$(CPYTHONROOT)/build/Python-$(PYVERSION)/

export TARGETINSTALLDIR=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)
export HOSTINSTALLDIR=$(PYODIDE_ROOT)/packages/.artifacts
export HOSTSITEPACKAGES=$(PYODIDE_ROOT)/packages/.artifacts/lib/python$(PYMAJOR).$(PYMINOR)/site-packages
export WASM_LIBRARY_DIR=$(PYODIDE_ROOT)/packages/.libs
export WASM_PKG_CONFIG_PATH=$(PYODIDE_ROOT)/packages/.libs/lib/pkgconfig

export PYTHONINCLUDE=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)/include/python$(PYMAJOR).$(PYMINOR)

# Use env variable if defined, otherwise fallback to './'
export PYODIDE_BASE_URL?=./

# The compression level used for zip files and wheels. When distributing via a
# CDN it's more efficient to keep this value to 0, and let the CDN perform the
# Brotli compression.
export PYODIDE_ZIP_COMPRESSION_LEVEL?=6

# For packages that depend on numpy.
# TODO: maybe move this somewhere else?
export NUMPY_LIB=$(HOSTSITEPACKAGES)/numpy/


# This environment variable is used for packages to detect if they are built
# for pyodide during build time
export PYODIDE=1
# This is the legacy environment variable used for the aforementioned purpose
export PYODIDE_PACKAGE_ABI=1

export DBGFLAGS_NODEBUG=-g0
export DBGFLAGS_WASMDEBUG=-g2
export DBGFLAGS_SOURCEMAPDEBUG=-g3
export DBG_LDFLAGS_SOURCEMAPDEBUG=-gseparate-dwarf

export DBGFLAGS=$(DBGFLAGS_NODEBUG)

ifdef PYODIDE_DEBUG
	export PYODIDE_SYMBOLS=1
	export PYODIDE_DEBUG_JS=1
	export PYODIDE_ASSERTIONS=1
endif

ifdef PYODIDE_SOURCEMAP
	# Debug with source maps (less useful than WASMDEBUG but easier if it helps)
	export DBGFLAGS=$(DBGFLAGS_SOURCEMAPDEBUG)
	export DBG_LDFLAGS=$(DBG_LDFLAGS_SOURCEMAPDEBUG)
else
	ifdef PYODIDE_SYMBOLS
		# Include debug symbols but no source maps (most useful)
		export DBGFLAGS=$(DBGFLAGS_WASMDEBUG)
	endif
endif

ifdef PYODIDE_ASSERTIONS
	EXTRA_CFLAGS+=" -DDEBUG_F"
endif


export OPTFLAGS=-O2
export CFLAGS_BASE=\
	$(OPTFLAGS) \
	$(DBGFLAGS) \
	-fPIC \
	$(EXTRA_CFLAGS)


export LDFLAGS_BASE=\
	$(OPTFLAGS) \
	$(DBGFLAGS) \
	$(DBG_LDFLAGS) \
	-s MODULARIZE=1 \
	-std=c++14 \
	-s LZ4=1 \
	-L $(CPYTHONROOT)/installs/python-$(PYVERSION)/lib/ \
	-s WASM_BIGINT \
	$(EXTRA_LDFLAGS)

export CXXFLAGS_BASE=\
	-std=c++14

export SIDE_MODULE_LDFLAGS=	$(LDFLAGS_BASE) -s SIDE_MODULE=1
export MAIN_MODULE_LDFLAGS= $(LDFLAGS_BASE) \
	-s MAIN_MODULE=1 \
	-s EXPORT_NAME="'_createPyodideModule'" \
	-s EXPORT_EXCEPTION_HANDLING_HELPERS \
	-s EXCEPTION_CATCHING_ALLOWED=['we only want to allow exception handling in side modules'] \
	-sEXPORTED_RUNTIME_METHODS='stackAlloc,stackRestore,stackSave' \
	-s DEMANGLE_SUPPORT=1 \
	-s USE_ZLIB \
	-s USE_BZIP2 \
	-s FORCE_FILESYSTEM=1 \
	-s TOTAL_MEMORY=20971520 \
	-s ALLOW_MEMORY_GROWTH=1 \
	-s EXPORT_ALL=1 \
	-s POLYFILL \
	-s MIN_SAFARI_VERSION=140000 \
	-s STACK_SIZE=5MB \
	-s AUTO_JS_LIBRARIES=0 \
	-s AUTO_NATIVE_LIBRARIES=0 \
	-s NODEJS_CATCH_EXIT=0 \
	-s NODEJS_CATCH_REJECTION=0 \
	\
	-lpython$(PYMAJOR).$(PYMINOR) \
	-lffi \
	-lstdc++ \
	-lidbfs.js \
	-lnodefs.js \
	-lproxyfs.js \
	-lworkerfs.js \
	-lwebsocket.js \
	-leventloop.js


export SIDE_MODULE_CXXFLAGS = $(CXXFLAGS_BASE)

export SIDE_MODULE_CFLAGS= $(CFLAGS_BASE) -I$(PYTHONINCLUDE)
export MAIN_MODULE_CFLAGS= $(CFLAGS_BASE) \
	-Wall \
	-Wno-warn-absolute-paths \
	-Werror=unused-variable \
	-Werror=sometimes-uninitialized \
	-Werror=int-conversion \
	-Werror=incompatible-pointer-types \
	-Werror=unused-result \
	-I$(PYTHONINCLUDE) \
	-s EXCEPTION_CATCHING_ALLOWED=['we only want to allow exception handling in side modules']

export STDLIB_MODULE_CFLAGS= $(SIDE_MODULE_CFLAGS) -I Include/ -I . -I Include/internal/

# For RUST
export CARGO_BUILD_TARGET=wasm32-unknown-emscripten
export CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_LINKER=emcc
export RUST_TOOLCHAIN=nightly-2022-06-26
export PYO3_CROSS_LIB_DIR=${CPYTHONINSTALL}/lib
export PYO3_CROSS_INCLUDE_DIR=${PYTHONINCLUDE}

# idealy we could automatically include all SIDE_MODULE_LDFLAGS here
export RUSTFLAGS= \
	-C link-arg=-sSIDE_MODULE=2 \
	-C link-arg=-sWASM_BIGINT \
	-Z link-native-libraries=no
.output_vars:
	set

export PYVERSION ?= 3.11.2
export PYODIDE_EMSCRIPTEN_VERSION ?= 3.1.32

export PLATFORM_TRIPLET=wasm32-emscripten
export SYSCONFIG_NAME=_sysconfigdata__emscripten_$(PLATFORM_TRIPLET)

# BASH_ENV tells bash to run pyodide_env.sh on startup, which sets various
# environment variables. The next line instructs make to use bash to run each
# command.
export BASH_ENV := $(PYODIDE_ROOT)/pyodide_env.sh
SHELL := /bin/bash

version_tuple := $(subst ., ,$(PYVERSION:v%=%))
export PYMAJOR=$(word 1,$(version_tuple))
export PYMINOR=$(word 2,$(version_tuple))
export PYMICRO=$(word 3,$(version_tuple))

export HOSTPYTHONROOT=$(shell python${PYMAJOR}.${PYMINOR} -c "import sys; print(sys.prefix)")
export HOSTPYTHON=$(HOSTPYTHONROOT)/bin/python$(PYMAJOR).$(PYMINOR)

export CPYTHONROOT=$(PYODIDE_ROOT)/cpython
export CPYTHONINSTALL=$(CPYTHONROOT)/installs/python-$(PYVERSION)
export CPYTHONLIB=$(CPYTHONINSTALL)/lib/python$(PYMAJOR).$(PYMINOR)
export SYSCONFIGDATA_DIR=$(CPYTHONINSTALL)/sysconfigdata/
export CPYTHONBUILD=$(CPYTHONROOT)/build/Python-$(PYVERSION)/

export TARGETINSTALLDIR=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)
export HOSTINSTALLDIR=$(PYODIDE_ROOT)/packages/.artifacts
export HOSTSITEPACKAGES=$(PYODIDE_ROOT)/packages/.artifacts/lib/python$(PYMAJOR).$(PYMINOR)/site-packages
export WASM_LIBRARY_DIR=$(PYODIDE_ROOT)/packages/.libs
export WASM_PKG_CONFIG_PATH=$(PYODIDE_ROOT)/packages/.libs/lib/pkgconfig

export PYTHONINCLUDE=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)/include/python$(PYMAJOR).$(PYMINOR)

# Use env variable if defined, otherwise fallback to './'
export PYODIDE_BASE_URL?=./

# The compression level used for zip files and wheels. When distributing via a
# CDN it's more efficient to keep this value to 0, and let the CDN perform the
# Brotli compression.
export PYODIDE_ZIP_COMPRESSION_LEVEL?=6

# For packages that depend on numpy.
# TODO: maybe move this somewhere else?
export NUMPY_LIB=$(HOSTSITEPACKAGES)/numpy/


# This environment variable is used for packages to detect if they are built
# for pyodide during build time
export PYODIDE=1
# This is the legacy environment variable used for the aforementioned purpose
export PYODIDE_PACKAGE_ABI=1

export DBGFLAGS_NODEBUG=-g0
export DBGFLAGS_WASMDEBUG=-g2
export DBGFLAGS_SOURCEMAPDEBUG=-g3
export DBG_LDFLAGS_SOURCEMAPDEBUG=-gseparate-dwarf

export DBGFLAGS=$(DBGFLAGS_NODEBUG)

ifdef PYODIDE_DEBUG
	export PYODIDE_SYMBOLS=1
	export PYODIDE_DEBUG_JS=1
	export PYODIDE_ASSERTIONS=1
endif

ifdef PYODIDE_SOURCEMAP
	# Debug with source maps (less useful than WASMDEBUG but easier if it helps)
	export DBGFLAGS=$(DBGFLAGS_SOURCEMAPDEBUG)
	export DBG_LDFLAGS=$(DBG_LDFLAGS_SOURCEMAPDEBUG)
else
	ifdef PYODIDE_SYMBOLS
		# Include debug symbols but no source maps (most useful)
		export DBGFLAGS=$(DBGFLAGS_WASMDEBUG)
	endif
endif

ifdef PYODIDE_ASSERTIONS
	EXTRA_CFLAGS+=" -DDEBUG_F"
endif


export OPTFLAGS=-O2
export CFLAGS_BASE=\
	$(OPTFLAGS) \
	$(DBGFLAGS) \
	-fPIC \
	$(EXTRA_CFLAGS)


export LDFLAGS_BASE=\
	$(OPTFLAGS) \
	$(DBGFLAGS) \
	$(DBG_LDFLAGS) \
	-s MODULARIZE=1 \
	-std=c++14 \
	-s LZ4=1 \
	-L $(CPYTHONROOT)/installs/python-$(PYVERSION)/lib/ \
	-s WASM_BIGINT \
	$(EXTRA_LDFLAGS)

export CXXFLAGS_BASE=\
	-std=c++14

export SIDE_MODULE_LDFLAGS=	$(LDFLAGS_BASE) -s SIDE_MODULE=1
export MAIN_MODULE_LDFLAGS= $(LDFLAGS_BASE) \
	-s MAIN_MODULE=1 \
	-s EXPORT_NAME="'_createPyodideModule'" \
	-s EXPORT_EXCEPTION_HANDLING_HELPERS \
	-s EXCEPTION_CATCHING_ALLOWED=['we only want to allow exception handling in side modules'] \
	-sEXPORTED_RUNTIME_METHODS='stackAlloc,stackRestore,stackSave' \
	-s DEMANGLE_SUPPORT=1 \
	-s USE_ZLIB \
	-s USE_BZIP2 \
	-s FORCE_FILESYSTEM=1 \
	-s TOTAL_MEMORY=20971520 \
	-s ALLOW_MEMORY_GROWTH=1 \
	-s EXPORT_ALL=1 \
	-s POLYFILL \
	-s MIN_SAFARI_VERSION=140000 \
	-s STACK_SIZE=5MB \
	-s AUTO_JS_LIBRARIES=0 \
	-s AUTO_NATIVE_LIBRARIES=0 \
	-s NODEJS_CATCH_EXIT=0 \
	-s NODEJS_CATCH_REJECTION=0 \
	\
	-lpython$(PYMAJOR).$(PYMINOR) \
	-lffi \
	-lstdc++ \
	-lidbfs.js \
	-lnodefs.js \
	-lproxyfs.js \
	-lworkerfs.js \
	-lwebsocket.js \
	-leventloop.js \
	\
	-lGL \
	-legl.js \
	-lwebgl.js \
	-lhtml5_webgl.js \
	-sGL_WORKAROUND_SAFARI_GETCONTEXT_BUG=0


export SIDE_MODULE_CXXFLAGS = $(CXXFLAGS_BASE)

export SIDE_MODULE_CFLAGS= $(CFLAGS_BASE) -I$(PYTHONINCLUDE)
export MAIN_MODULE_CFLAGS= $(CFLAGS_BASE) \
	-Wall \
	-Wno-warn-absolute-paths \
	-Werror=unused-variable \
	-Werror=sometimes-uninitialized \
	-Werror=int-conversion \
	-Werror=incompatible-pointer-types \
	-Werror=unused-result \
	-I$(PYTHONINCLUDE) \
	-s EXCEPTION_CATCHING_ALLOWED=['we only want to allow exception handling in side modules']

export STDLIB_MODULE_CFLAGS= $(SIDE_MODULE_CFLAGS) -I Include/ -I . -I Include/internal/

# For RUST
export CARGO_BUILD_TARGET=wasm32-unknown-emscripten
export CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_LINKER=emcc
export RUST_TOOLCHAIN=nightly-2022-06-26
export PYO3_CROSS_LIB_DIR=${CPYTHONINSTALL}/lib
export PYO3_CROSS_INCLUDE_DIR=${PYTHONINCLUDE}

# idealy we could automatically include all SIDE_MODULE_LDFLAGS here
export RUSTFLAGS= \
	-C link-arg=-sSIDE_MODULE=2 \
	-C link-arg=-sWASM_BIGINT \
	-Z link-native-libraries=no
.output_vars:
	set
