output: refactor frame submission API

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

examples/tablet.c

@@ -86,7 +86,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	int32_t width, height;
 	wlr_output_effective_resolution(wlr_output, &width, &height);
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(sample->renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(sample->renderer, (float[]){0.25f, 0.25f, 0.25f, 1});
 
@@ -129,7 +129,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(sample->renderer);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 	sample->last_frame = now;
 }