Expose the map renderer type to the public API

In the case  where the map decides itself which renderer to use. It can
be convenient for the developer to have knowledge of the choice made.

src/ol/renderer/maprenderer.js

@@ -163,6 +163,12 @@ ol.renderer.Map.prototype.getMap = function() {
 };
 
 
+/**
+ * @return {string} Type
+ */
+ol.renderer.Map.prototype.getType = goog.abstractMethod;
+
+
 /**
  * @param {string} layerKey Layer key.
  * @return {ol.renderer.Layer} Layer renderer.