stage: Call ClutterStageWindow::unrealize() on dispose

Since we realize on creation we need to unrealize on destruction. This
makes sure that the ClutterStageWindow implementation can tear down any
resource set up during the realization phase.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2559

clutter/clutter-stage.c

@@ -1176,6 +1176,7 @@ clutter_stage_dispose (GObject *object)
     {
       CLUTTER_NOTE (BACKEND, "Disposing of the stage implementation");
 
+      _clutter_stage_window_unrealize (priv->impl);
       g_object_unref (priv->impl);
       priv->impl = NULL;
     }