window-actor: Consider needs_destroy in is_destroyed()

According to the documentation, the method returns "whether the X window
that the actor was displaying has been destroyed" - that is very much
true when we delay the actual actor destruction for a destroy animation,
so update the method accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=735927

src/compositor/meta-window-actor.c

@@ -865,7 +865,7 @@ meta_window_actor_get_surface (MetaWindowActor *self)
 gboolean
 meta_window_actor_is_destroyed (MetaWindowActor *self)
 {
-  return self->priv->disposed;
+  return self->priv->disposed || self->priv->needs_destroy;
 }
 
 static gboolean