Refactor inter-context-canvas api

[almarklein]

Closes #20

This will break compatibility with wgpu-py. So let's keep this draft for now.

Stuff that allows use of other contexts:

• Rename _rc_get_present_info() -> _rc_get_present_methods()
• Rename the "image" present method to "bitmap".
• Document what _rc_get_present_info() can/should return.
• Refactor canvas.get_context() mechanics. Can now load a context from anywhere.
• Document the API that a context should have in order to play nice with the above.

Supporting get_context("bitmap"):

• Implement a simple BitmapRenderingContext.
• Implement a BitmapToScreenAdapter to help contexts like the above to render to screen for canvases that only support the screen present-method.

Other:

• Show example-snippets in docs and readme
• Examples.
• Tests.

This needs to go at the same time as pygfx/wgpu-py#642, because it's mutually dependent. Interesting 🤔

[almarklein]

This docs page is I think a good summary of what I'm trying to achieve here: https://rendercanvas--21.org.readthedocs.build/21/contextapi.html