Allow styles to configure a custom renderer

Two new examples show how custom renderers can be used to render text along
paths, and to declutter labels using 3rd party libraries.

src/ol/render/feature.js

@@ -43,6 +43,12 @@ ol.render.Feature = function(type, flatCoordinates, ends, properties, id) {
    */
   this.flatCoordinates_ = flatCoordinates;
 
+  /**
+   * @private
+   * @type {Array.<number>|Array.<Array.<number>>|Array.<Array.<Array.<number>>>}
+   */
+  this.renderCoordinates_ = null;
+
   /**
    * @private
    * @type {Array.<number>|Array.<Array.<number>>}
@@ -111,6 +117,18 @@ ol.render.Feature.prototype.getOrientedFlatCoordinates = function() {
 };
 
 
+/**
+ * @return {Array.<number>|Array.<Array.<number>>|Array.<Array.<Array.<number>>>}
+ *     Render coordinates.
+ */
+ol.render.Feature.prototype.getRenderCoordinates = function() {
+  if (!this.renderCoordinates_) {
+    this.renderCoordinates_ = [];
+  }
+  return this.renderCoordinates_;
+};
+
+
 /**
  * @return {Array.<number>} Flat coordinates.
  */