meta-surface-actor-x11: Detatch the pixmap in window_decorated_notify

The window can change its decoration without changing its size.

https://bugzilla.gnome.org/show_bug.cgi?id=738146

Found by Jasper St. Pierre <jstpierre@mecheye.net>

src/compositor/meta-surface-actor-x11.c

@@ -383,6 +383,7 @@ window_decorated_notify (MetaWindow *window,
 {
   MetaSurfaceActorX11 *self = META_SURFACE_ACTOR_X11 (user_data);
 
+  detach_pixmap (self);
   free_damage (self);
   create_damage (self);
 }