workspace: End zooming when starting a window drag

The mouse-wheel zooming "easter egg" breaks horribly when you
drag a window, due to ugly lightbox reparenting tricks it uses.

For now, just end any zoom before we drag the window around.

https://bugzilla.gnome.org/show_bug.cgi?id=649632

js/ui/workspace.js

@@ -296,6 +296,9 @@ WindowClone.prototype = {
     },
 
     _onDragBegin : function (draggable, time) {
+        if (this._zooming)
+            this._zoomEnd();
+
         [this.dragOrigX, this.dragOrigY] = this.actor.get_position();
         this.dragOrigScale = this.actor.scale_x;
         this.inDrag = true;