Make pixel projection handling work in compiled mode

The compiler seems to have trouble with functions that can run with
different 'this' types. By moving the units handling to the callback, this
issue can be avoided.

src/ol/vectortile.js

@@ -161,6 +161,7 @@ ol.VectorTile.prototype.load = function() {
 
 /**
  * @param {Array.<ol.Feature>} features Features.
+ * @api
  */
 ol.VectorTile.prototype.setFeatures = function(features) {
   this.features_ = features;