Add a removePreRenderFunction method to ol.Map

This will remove the pre-render function from preRenderFunctions_ array if
found.

src/ol/map.js

@@ -326,6 +326,15 @@ ol.Map.prototype.addPreRenderFunctions = function(preRenderFunctions) {
 };
 
 
+/**
+ * @param {ol.PreRenderFunction} preRenderFunction Pre-render function.
+ * @return {boolean} Whether the preRenderFunction has been found and removed.
+ */
+ol.Map.prototype.removePreRenderFunction = function(preRenderFunction) {
+  return goog.array.remove(this.preRenderFunctions_, preRenderFunction);
+};
+
+
 /**
  *
  * @inheritDoc