WindowManager: clean up scale_y when overwrite the map animation

If we overwrite a map animation (for example because the actor is now destroyed),
we need to complete it first, otherwise it starts off from a random
middle point. This is the same treatment opacity gets for normal windows.

https://bugzilla.gnome.org/show_bug.cgi?id=690241

js/ui/windowManager.js

@@ -420,6 +420,7 @@ const WindowManager = new Lang.Class({
         if (this._removeEffect(this._mapping, actor)) {
             Tweener.removeTweens(actor);
             actor.opacity = 255;
+            actor.scale_y = 1;
             shellwm.completed_map(actor);
         }
     },