Move compositor-stack handling to MetaStackTracker

In order to properly track the stacking order for override-redirect
windows, move meta_compositor_sync_stack() call into MetaStackTracker.
In the new location, we sync the stack as a before-redraw idle function,
rather then using the freeze-thaw facilities of MetaStack. This is
simpler, and also properly compresses multiple stack changes on
notifications received from the X server.

http://bugzilla.gnome.org/show_bug.cgi?id=585984

src/core/screen.c

@@ -953,9 +953,8 @@ meta_screen_composite_all_windows (MetaScreen *screen)
     meta_compositor_add_window (display->compositor, tmp->data);
   g_slist_free (windows);
   
-  /* trigger a stack_sync_to_server: */
-  meta_stack_freeze (screen->stack);
-  meta_stack_thaw (screen->stack);
+  /* initialize the compositor's view of the stacking order */
+  meta_stack_tracker_sync_stack (screen->stack_tracker);
 #endif
 }