output: Implement request_state event listener

wlroots backends no longer change state on their own, and instead send a
request_state event. Monitor this event and apply any state we receive.

output.h

@@ -13,6 +13,7 @@ struct cg_output {
 	struct wlr_scene_output *scene_output;
 
 	struct wl_listener commit;
+	struct wl_listener request_state;
 	struct wl_listener destroy;
 	struct wl_listener frame;