window-actor: Fix check to clip beneath the window

We want to clip it away if 1) The window is fully opaque or
2) If it's translucent but has a frame (as explained in the comment
above). The code didn't quite match and we were only applying it on
case #2.

Case #1 is far more common, and saves us from pushing some drawing
that we know will be covered in the end.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/698

src/compositor/meta-window-actor.c

@@ -647,7 +647,10 @@ clip_shadow_under_window (MetaWindowActor *self)
   MetaWindowActorPrivate *priv =
     meta_window_actor_get_instance_private (self);
 
-  return is_non_opaque (self) && priv->window->frame;
+  if (priv->window->frame)
+    return TRUE;
+
+  return !is_non_opaque (self);
 }
 
 static void