This is necessary for direct scan-out and other upcoming features. This patch
changes the output API to look like the wl_surface API.

Outputs now have some double-buffered state: the frame to be submitted
(currently only wlr_renderer frames are supported) and the damaged region.
To attach a pending frame, use wlr_output_attach_render. To set the pending
damaged region, use wlr_output_set_damage.

To submit the pending state, call wlr_output_commit. This will submit the
pending frame to the backend.

To migrate from the old API to the new one:

- Replace wlr_output_make_current calls by wlr_output_attach_render
- Replace wlr_output_swap_buffers calls by wlr_output_set_damage and
  wlr_output_commit