dnd: Set the scale on the drag actor

The actor is the same as the drag actor in this case, but we tend to
set properties on the drag actor and get them from the actor elsewhere
in this codepath. Make this consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=697504

js/ui/dnd.js

@@ -302,8 +302,8 @@ const _Draggable = new Lang.Class({
             // Set the actor's scale such that it will keep the same
             // transformed size when it's reparented to the uiGroup
             let [scaledWidth, scaledHeight] = this.actor.get_transformed_size();
-            this.actor.set_scale(scaledWidth / this.actor.width,
-                                 scaledHeight / this.actor.height);
+            this._dragActor.set_scale(scaledWidth / this.actor.width,
+                                      scaledHeight / this.actor.height);
         }
 
         this._dragOrigOpacity = this._dragActor.opacity;