background: Set the correct index on images

The pattern is underneath, so we need to add one. It turns out
that ClutterGroup doesn't care about that, but we need to do this
to remove deprecations in mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=695882

js/ui/background.js

@@ -356,7 +356,10 @@ const Background = new Lang.Class({
 
         let actor = new Meta.BackgroundActor();
         actor.content = content;
-        this.actor.insert_child_at_index(actor, index);
+
+        // The background pattern is the first actor in
+        // the group, and all images should be above that.
+        this.actor.insert_child_at_index(actor, index + 1);
 
         this._images[index] = actor;
         this._watchCacheFile(filename);