Make tileCoordTransform configurable

Instead of using the static createOriginTopLeftTileCoordTransform
function, the correct transform is now a non-API config option of the tile
grid.

src/ol/source/xyzsource.js

@@ -41,8 +41,7 @@ ol.source.XYZ = function(options) {
    * @private
    * @type {ol.TileCoordTransformType}
    */
-  this.tileCoordTransform_ =
-      ol.tilegrid.createOriginTopLeftTileCoordTransform(tileGrid);
+  this.tileCoordTransform_ = tileGrid.createTileCoordTransform();
 
   if (goog.isDef(options.tileUrlFunction)) {
     this.setTileUrlFunction(options.tileUrlFunction);