layout: Don't bother updating the regions in the greeter

The greeter is always full-screen, so this should never matter.

https://bugzilla.gnome.org/show_bug.cgi?id=694227

js/ui/layout.js

@@ -829,6 +829,9 @@ const LayoutManager = new Lang.Class({
     },
 
     _queueUpdateRegions: function() {
+        if (Main.sessionMode.isGreeter)
+            return;
+
         if (!this._updateRegionIdle && !this._freezeUpdateCount)
             this._updateRegionIdle = Mainloop.idle_add(Lang.bind(this, this._updateRegions),
                                                        Meta.PRIORITY_BEFORE_REDRAW);