wlr_scene: Track output damage separately

The damage ring is only supposed to keep track of damage within
the swapchain

include/wlr/types/wlr_scene.h

@@ -204,6 +204,8 @@ struct wlr_scene_output {
 
 	// private state
 
+	pixman_region32_t pending_commit_damage;
+
 	uint8_t index;
 	bool prev_scanout;