Introduce wlr_raster

Incorporates some ideas from: #3450

This PR is a big step forward in terms of multigpu as we are removing assumptions about what renderers might be used. For now, multigpu is handled completely by the backend in which case wlr_raster leaves that all alone. Future plans will lift responsibility away from backends for multigpu and allow for different renderers to run on different outputs so that we can stop doing the worst possible thing: Funneling everything through an arbitrary GPU for compositing then blitting every frame for other outputs.

This fixes wlr_scene partial texture upload.

Breaking changes:

• wlr_client_buffer has been removed and replaced with wlr_surface.raster property
• wlr_scene_buffer has been replaced with a raster variant
  • Not much has changed, just a lot of renaming