canvas.go
func PopClip() {
	if len(canvas.clipStack) == 0 {
		return
	}
	c := canvas.clipStack[len(canvas.clipStack)-1]
	canvas.clipStack = canvas.clipStack[:len(canvas.clipStack)-1]
	SetClipRect(c.x, c.y, c.w, c.h)
}
func PopClip() {
	if len(canvas.clipStack) == 0 {
		return
	}
	c := canvas.clipStack[len(canvas.clipStack)-1]
	canvas.clipStack = canvas.clipStack[:len(canvas.clipStack)-1]
	SetClipRect(c.x, c.y, c.w, c.h)
}
// PopClip restores saved with PushClip clipping window