Add wlr_output_layer, take 2

This is based on previous work 1 2.

This new API allows compositors to display buffers without needing to perform rendering operations. This API can be implemented on Wayland using subsurfaces and on DRM using KMS planes.

Compared to 1, this approach leverages wlr_addon_set to let backends attach their own private state to layers, and removes the pending state (necessary for interop with wlr_output_commit_state()).