Custom wgpu rendering

[nicoburns]

This issue tracks the ability to render custom content (rendered with wgpu) into a Blitz scene.

Notes

• The Vello renderer allows for a custom wgpu texture to be rendered in a Vello scene using: https://docs.rs/vello/latest/vello/struct.Renderer.html#method.override_image. To use this API, one creates a dummy peniko::Image of the correct dimensions, call this API with the texture you wish to use. You can then render the texture by drawing calling Scene::draw_image with the dummy image (yes this API is dumb, but it works so 🤷 )
• We need to work out an API for how to do it. The web typically uses the <canvas> element for custom rendered content. It would probably make sense for us to do the same here. Either the <canvas> element should own a wgpu Texture and we need some way to get a reference to it. Or we should crate our own wgpu texture, and we need to some way to pass it to the <canvas> - probably makes sense for the <canvas> to own the texture?