scene/output: disable hardware cursors when idle

When the cursor plane is enabled, a lot of hardware will drain more
power. If the screen contents don't change, perform one final render
pass to blend everything onto a single plane.

include/wlr/types/wlr_scene.h

@@ -204,6 +204,8 @@ struct wlr_scene_output {
 	struct wl_list damage_highlight_regions;
 
 	struct wl_array render_list;
+
+	struct wl_event_source *idle_timer;
 };
 
 /** A layer shell scene helper */