Add getSource getter in ol.source.ImageVector

src/ol/source/imagevectorsource.exports

@@ -1 +1,2 @@
 @exportSymbol ol.source.ImageVector
+@exportProperty ol.source.ImageVector.prototype.getSource

src/ol/source/imagevectorsource.js

@@ -163,6 +163,15 @@ ol.source.ImageVector.prototype.forEachFeatureAtPixel =
 };
 
 
+/**
+ * @return {ol.source.Vector} Source.
+ * @todo stability experimental
+ */
+ol.source.ImageVector.prototype.getSource = function() {
+  return this.source_;
+};
+
+
 /**
  * @param {ol.Coordinate} center Center.
  * @param {number} resolution Resolution.