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/style/style.js

@@ -50,6 +50,12 @@ ol.style.Style = function(opt_options) {
    */
   this.image_ = options.image !== undefined ? options.image : null;
 
+  /**
+   * @private
+   * @type {ol.StyleRenderFunction|null}
+   */
+  this.renderer_ = options.renderer !== undefined ? options.renderer : null;
+
   /**
    * @private
    * @type {ol.style.Stroke}
@@ -92,6 +98,28 @@ ol.style.Style.prototype.clone = function() {
 };
 
 
+/**
+ * Get the custom renderer function that was configured with
+ * {@link #setRenderer} or the `renderer` constructor option.
+ * @return {ol.StyleRenderFunction|null} Custom renderer function.
+ * @api
+ */
+ol.style.Style.prototype.getRenderer = function() {
+  return this.renderer_;
+};
+
+
+/**
+ * Sets a custom renderer function for this style. When set, `fill`, `stroke`
+ * and `image` optins of the style will be ignored.
+ * @param {ol.StyleRenderFunction|null} renderer Custom renderer function.
+ * @api
+ */
+ol.style.Style.prototype.setRenderer = function(renderer) {
+  this.renderer_ = renderer;
+};
+
+
 /**
  * Get the geometry to be rendered.
  * @return {string|ol.geom.Geometry|ol.StyleGeometryFunction}