window-actor: Add API to get a MetaWindowActor from a ClutterActor

Make it so it returns the closest ancestry MetaWindowActor if it
is a MetaSurfaceActor.
We need this for Wayland subsurfaces, so we can support actions like
Meta+Drag on them.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/604

src/compositor/meta-window-actor-private.h

@@ -86,5 +86,6 @@ void meta_window_actor_assign_surface_actor (MetaWindowActor  *self,
                                              MetaSurfaceActor *surface_actor);
 
 MetaWindowActor *meta_window_actor_from_window (MetaWindow *window);
+MetaWindowActor *meta_window_actor_from_actor (ClutterActor *actor);
 
 #endif /* META_WINDOW_ACTOR_PRIVATE_H */

src/compositor/meta-window-actor.c

@@ -2035,3 +2035,21 @@ screen_cast_window_iface_init (MetaScreenCastWindowInterface *iface)
   iface->capture_into = meta_window_actor_capture_into;
   iface->has_damage = meta_window_actor_has_damage;
 }
+
+MetaWindowActor *
+meta_window_actor_from_actor (ClutterActor *actor)
+{
+  if (!META_IS_SURFACE_ACTOR (actor))
+    return NULL;
+
+  do
+    {
+      actor = clutter_actor_get_parent (actor);
+
+      if (META_IS_WINDOW_ACTOR (actor))
+        return META_WINDOW_ACTOR (actor);
+    }
+  while (actor != NULL);
+
+  return NULL;
+}