Use the last child of the container only if loading was aborted

Otherwise we may pick a tile that is already half way through the
transition, which will cause the backbuffer to go away too early.

lib/OpenLayers/Layer/Grid.js

@@ -1116,7 +1116,8 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
                         this.removeBackBuffer();
                     } else {
                         // wait until transition has ended or delay has passed
-                        this._transitionElement = this.div.lastChild;
+                        this._transitionElement = aborted ?
+                            this.div.lastChild : tile.imgDiv;
                         var transitionendEvents = this.transitionendEvents;
                         for (var i=transitionendEvents.length-1; i>=0; --i) {
                             OpenLayers.Event.observe(this._transitionElement,