69b2747ca3
This implements platform views in Skwasm. There are a number of substantial changes rolled up in this change, including: * Reworked the rendering system to use multiple canvases with a single WebGL context, via ImageBitmap rendering. * Reworked our object management and bindings to use the `__externref_t` construct in C code. * Upgraded emscripten to a much newer version * Generified skwasm's scene builder to be able to work with any renderer that can produce `ImageBitmap` objects from Pictures, and whose `Canvas`, `Picture` and `ImageFilter` objects conform to `SceneCanvas`/`ScenePicture`/`SceneImageFilter` For platform views themselves, most stuff is implemented except for clipping. I plan on doing that in a subsequent change.