### Description

Adds opencv-python and ffmpeg.

### Status

opencv-python is modularized with a bunch of modules, I'll keep updating the status.

- [x] Core library
- [x] JPEG
- [x] PNG
- [x] WEBP
- [x] Image processing
- [x] video processing (FFMPEG)
- [x] objectdetect
- [x] dnn
- [x] ml
- [x] calib3d
- [x] feature2d
- [x] highgui (built, but nonsense)
- [x] photo
- [x] stitching
- [x] flann
- contrib algorithms?
- extra modules?

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

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

Resolve #381
Resolve #1627
PKG Add opencv-python and ffmpeg. ### Description

Adds opencv-python and ffmpeg.

### Status

opencv-python is modularized with a bunch of modules, I'll keep updating the status.

- [x] Core library
- [x] JPEG
- [x] PNG
- [x] WEBP
- [x] Image processing
- [x] video processing (FFMPEG)
- [x] objectdetect
- [x] dnn
- [x] ml
- [x] calib3d
- [x] feature2d
- [x] highgui (built, but nonsense)
- [x] photo
- [x] stitching
- [x] flann
- contrib algorithms?
- extra modules?

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

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

Resolve #381
Resolve #1627
// The point is to make a file that works with JavaScript analysis tools like
// JsDoc and LGTM. They want to parse the file as JavaScript. Thus, it's key
// that included js files should parse as valid JavaScript. `JS_FILE` is a
// specially designed macro to allow us to do this. We need to look like a
// function call to JavaScript parsers. The easiest way to get it to parse is to
// make the macro argument look like a JavaScript anonymous function, which we
// do with `()=>{`. However, `()=>{` is an invalid C string so the macro needs
// to remove it. We put `()=>{0,0;`, JS_FILE removes everything up to
// the comma and replace it with a single open brace.
//

#define UNPAIRED_OPEN_BRACE {
#define UNPAIRED_CLOSE_BRACE } // Just here to help text editors pair braces up
#define JS_FILE(func_name, a, args...)                                         \
  EM_JS_NUM(                                                                   \
    int, func_name, (), UNPAIRED_OPEN_BRACE { args return 0; })

// A macro to allow us to add code that is only intended to influence JsDoc
// output, but shouldn't end up in generated code.
#define FOR_JSDOC_ONLY(x)

// The point is to make a file that works with JavaScript analysis tools like
// JsDoc and LGTM. They want to parse the file as JavaScript. Thus, it's key
// that included js files should parse as valid JavaScript. `JS_FILE` is a
// specially designed macro to allow us to do this. We need to look like a
// function call to JavaScript parsers. The easiest way to get it to parse is to
// make the macro argument look like a JavaScript anonymous function, which we
// do with `()=>{`. However, `()=>{` is an invalid C string so the macro needs
// to remove it. We put `()=>{0,0;`, JS_FILE removes everything up to
// the comma and replace it with a single open brace.
//

#define UNPAIRED_OPEN_BRACE {
#define UNPAIRED_CLOSE_BRACE } // Just here to help text editors pair braces up
#define JS_FILE(func_name, a, args...)                                         \
  EM_JS_NUM(                                                                   \
    int, func_name, (), UNPAIRED_OPEN_BRACE { args return 0; })

// A macro to allow us to add code that is only intended to influence JsDoc
// output, but shouldn't end up in generated code.
#define FOR_JSDOC_ONLY(x)
