scene: add user data pointer to wlr_scene_node

This allows compositors to attach arbitrary data to the scene-graph
nodes.

include/wlr/types/wlr_scene.h

@@ -48,6 +48,8 @@ struct wlr_scene_node {
 	struct {
 		struct wl_signal destroy;
 	} events;
+
+	void *data;
 };
 
 /** The root scene-graph node. */