wlr_scene: Hook up HIGHLIGHT logic

This will display red translucent rectangles on the screen regions that
have been damaged. These rectangles will fade out over the span of 250
msecs. If the area is damaged again while the region is fading out,
the timer is reset.

Let's also disable direct scan out when this option is enabled, or else
we won't be able to render the highlight damage regions.

include/wlr/types/wlr_scene.h

@@ -91,6 +91,7 @@ struct wlr_scene {
 	struct wl_listener presentation_destroy;
 
 	enum wlr_scene_debug_damage_option debug_damage_option;
+	struct wl_list damage_highlight_regions;
 };
 
 /** A sub-tree in the scene-graph. */