Investigate performance impact of vertex array and framebuffer rebinding

[leod]

Currently, for convenience, we just have a single global VAO and a single global framebuffer whose attachments are updated in each draw call. We need to check if this has performance implications.

Note that we do not want to expose these objects to the user-facing API, since having to maintain them makes the API more cumbersome. However, we could consider to have an internal caching layer. IIRC, glium does this for vertex arrays.

[leod]

Note that Mozilla's WebGL best practices say this:

• Avoid invalidating FBO attachment bindings
• Avoid changing VAO attachments (vertexAttribPointer, disable/enableVertexAttribArray)

I'm not sure what the exact overhead of this is. I'm fine with a small amount of per-draw-call overhead, but pipeline stalls must be avoided.