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/geom/simplegeometry.js

@@ -41,6 +41,12 @@ ol.geom.SimpleGeometry = function() {
    */
   this.flatCoordinates = null;
 
+  /**
+   * @private
+   * @type {Array.<number>|Array.<Array.<number>>|Array.<Array.<Array.<number>>>}
+   */
+  this.renderCoordinates_ = null;
+
 };
 ol.inherits(ol.geom.SimpleGeometry, ol.geom.Geometry);
 
@@ -141,6 +147,18 @@ ol.geom.SimpleGeometry.prototype.getLayout = function() {
 };
 
 
+/**
+ * @return {Array.<number>|Array.<Array.<number>>|Array.<Array.<Array.<number>>>}
+ *     Render coordinates.
+ */
+ol.geom.SimpleGeometry.prototype.getRenderCoordinates = function() {
+  if (!this.renderCoordinates_) {
+    this.renderCoordinates_ = [];
+  }
+  return this.renderCoordinates_;
+};
+
+
 /**
  * @inheritDoc
  */