Magnifier: clip the UI group clone to the allocation

Without clipping, we show parts of the message tray and workspace
thumbnails that are normally clipped by the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=724305

js/ui/magnifier.js

@@ -1208,7 +1208,8 @@ const ZoomRegion = new Lang.Class({
 
         // Clone the group that contains all of UI on the screen.  This is the
         // chrome, the windows, etc.
-        this._uiGroupClone = new Clutter.Clone({ source: Main.uiGroup });
+        this._uiGroupClone = new Clutter.Clone({ source: Main.uiGroup,
+                                                 clip_to_allocation: true });
         mainGroup.add_actor(this._uiGroupClone);
 
         // Add either the given mouseSourceActor to the ZoomRegion, or a clone of