WIP Format API

[ids1024]

Very initial implementation for #98. No conversion support, and Wayland only, but allows listing available formats and selecting which to use. Seems good to get a start on this and see how things work.

It should be easy to add basic format conversion support swapping the first and third component. I'm not sure exactly about alpha. If a platform always uses alpha, a format like BGRX can't be supported without conversion (setting the fourth component to 255). If a platform doesn't support alpha, formats with alpha probably shouldn't be supported even with conversion? Instead of just listing Format, it could list structs with that and a native: bool field to indicate that the format is native and doesn't require conversion...

[ids1024]

Handling conversion isn't hard (unless we add more unusual formats), though I'm thinking about how I'd want to integrate it.

• A conversion API that is internally used within each backend and called into by the backend's implementation of (at least) pixels/pixels_mut/present
• Code in toplevel pixels/present/etc. that either calls to the conversion code, or the backend directly
• A new variant of BufferDispatch that does conversion first, then calls through to the backend.