Add ol.source.Tile#getGutter

src/ol/source/tilesource.js

@@ -106,6 +106,14 @@ ol.source.Tile.prototype.findLoadedTiles = function(loadedTilesByZ,
 };
 
 
+/**
+ * @return {number} Gutter.
+ */
+ol.source.Tile.prototype.getGutter = function() {
+  return 0;
+};
+
+
 /**
  * @param {number} z Z.
  * @param {number} x X.