box: Tweak the depth sorting function

By changing the depth sorting function we can achieve the same effect as
ClutterGroup but without the costly append+sort.

http://bugzilla.openedhand.com/show_bug.cgi?id=1988

clutter/clutter-box.c

@@ -119,7 +119,7 @@ sort_by_depth (gconstpointer a,
   if (depth_a < depth_b)
     return -1;
 
-  if (depth_a > depth_b)
+  if (depth_a >= depth_b)
     return 1;
 
   return 0;