Make render methods ol vector layer and image vector source exportable

This addresses
openlayers#2684 (comment)

Author: ahocevar

src/ol/layer/vectorlayer.js

@@ -101,6 +101,7 @@ ol.layer.Vector.prototype.getStyleFunction = function() {
 /**
  * Renders the layer; rendering will effectively occur at the next browser
  * animation frame. Call this for style changes to take effect.
+ * @api
  */
 ol.layer.Vector.prototype.render = function() {
   this.dispatchChangeEvent();

src/ol/source/imagevectorsource.js

@@ -257,6 +257,7 @@ ol.source.ImageVector.prototype.handleSourceChange_ = function() {
 /**
  * Renders the image; rendering will effectively occur at the next browser
  * animation frame. Call this for style changes to take effect.
+ * @api
  */
 ol.source.ImageVector.prototype.render =
     function() {