Fix #6972. Setting game objects ignoreDestroy to true no longer c…

…auses infinite loop bug when `shutdown` is called.

src/gameobjects/DisplayList.js

@@ -235,12 +235,15 @@ var DisplayList = new Class({
     shutdown: function ()
     {
         var list = this.list;
+        var i = list.length;
 
-        while (list.length)
+        while (i--)
         {
-            list[0].destroy(true);
+            list[i].destroy(true);
         }
 
+        list.length = 0;
+
         this.events.off(SceneEvents.SHUTDOWN, this.shutdown, this);
     },