Only listen once for tile events

Assuming browser garbage collection cannot happen while we have listeners in the global registry, we need to listenOnce to avoid memory leaks with the tile queue.

src/ol/tilequeue.js

@@ -174,7 +174,7 @@ ol.TileQueue.prototype.loadMoreTiles = function() {
   var tile;
   while (this.heap_.length > 0 && this.tilesLoading_ < this.maxTilesLoading_) {
     tile = /** @type {ol.Tile} */ (this.dequeue_());
-    goog.events.listen(tile, goog.events.EventType.CHANGE,
+    goog.events.listenOnce(tile, goog.events.EventType.CHANGE,
         this.handleTileChange, false, this);
     tile.load();
     ++this.tilesLoading_;