Don't move hidden windows to the desktop layer

Putting hidden windows in the desktop layer is pointless - in
the desktop layer isn't necessary below all visible windows,
and we are hiding the windows by other means. And the movement
isn't reliable because nothing sets stack->needs_relayer, so
windows can get stuck in the desktop layer after being
rehidden.

http://bugzilla.gnome.org/show_bug.cgi?id=587251

src/core/stack.c

@@ -241,12 +241,6 @@ get_standalone_layer (MetaWindow *window)
   MetaStackLayer layer;
   gboolean focused_transient = FALSE;
 
-  if (window->hidden)
-    {
-      layer = META_LAYER_DESKTOP;
-      return layer;
-    }
-
   switch (window->type)
     {
     case META_WINDOW_DESKTOP: