Re-add the ol.render.Feature#getGeometry() method

This method does have value for users that want to switch between ol.Feature
and ol.render.Feature and still be able to use #getGeometry().getType() in a
style function to determine the feature geometry's type.

src/ol/render/feature.js

@@ -121,6 +121,7 @@ ol.render.Feature.prototype.getFlatCoordinates =
 /**
  * Get the feature for working with its geometry.
  * @return {ol.render.Feature} Feature.
+ * @api
  */
 ol.render.Feature.prototype.getGeometry = function() {
   return this;