Screenshot, screencast, and the ability to attach an additional offscreen backend

[almarklein]

@Vipitis mentioned this somewhere, to allow recording video while using any backend.

This also covers taking screenshots, e.g. first requested in #30

The idea is to allow attaching one (or more?) offscreen backends to an existing canvas. Pretty simple for bitmap context (unless we allow resizing) and also doable for wgpu contexts, I think.