Rewrite Web platform

[K0bin]

I'm using too much C++ code anyway to target the pure Rust WASM export + Emscripten handles a lot of annoying things for me (and works better with threads).

I'll also have to rewrite the WebGL graphics backend, either using OpenGL (ES) directly or using WebGPU which has a WebGL backend itself. I'm leaning towards the latter option to be able to get rid of OpenGL as soon as possible.