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/vectorcontext.js

@@ -13,6 +13,16 @@ ol.render.VectorContext = function() {
 };
 
 
+/**
+ * Render a geometry with a custom renderer.
+ *
+ * @param {ol.geom.SimpleGeometry} geometry Geometry.
+ * @param {ol.Feature|ol.render.Feature} feature Feature.
+ * @param {Function} renderer Renderer.
+ */
+ol.render.VectorContext.prototype.drawCustom = function(geometry, feature, renderer) {};
+
+
 /**
  * Render a geometry.
  *