Write modifications back to the original layer

With the feature's getOriginal() and setOriginal() methods, an
undo tree can be maintained now.

src/ol/feature.js

@@ -42,6 +42,13 @@ ol.Feature = function(opt_values) {
    */
   this.geometryName_;
 
+  /**
+   * Original of this feature when it was modified.
+   * @type {ol.Feature}
+   * @private
+   */
+  this.original_ = null;
+
   /**
    * The render intent for this feature.
    * @type {ol.layer.VectorLayerRenderIntent|string}
@@ -101,6 +108,15 @@ ol.Feature.prototype.getGeometry = function() {
 };
 
 
+/**
+ * Get the original of this feature when it was modified.
+ * @return {ol.Feature} Original.
+ */
+ol.Feature.prototype.getOriginal = function() {
+  return this.original_;
+};
+
+
 /**
  * Get any symbolizers set directly on the feature.
  * @return {Array.<ol.style.Symbolizer>} Symbolizers (or null if none).
@@ -176,6 +192,15 @@ ol.Feature.prototype.setGeometry = function(geometry) {
 };
 
 
+/**
+ * Set the original of this feature when it was modified.
+ * @param {ol.Feature} original Original.
+ */
+ol.Feature.prototype.setOriginal = function(original) {
+  this.original_ = original;
+};
+
+
 /**
  * Gets the renderIntent for this feature.
  * @return {string} Render intent.