Add layerName option for WMSGetFeatureInfo format

This option allows the format to read only features of a
given layer. This is useful if you wish to make a single
query to a WMS server with multiple layers in it.

externs/olx.js

@@ -2217,6 +2217,24 @@ olx.format.WKTOptions;
 olx.format.WKTOptions.prototype.splitCollection;
 
 
+/**
+ * @typedef {{
+ *     layers: (Array.<string>|undefined)
+ * }}
+ * @api
+ */
+olx.format.WMSGetFeatureInfoOptions;
+
+
+/**
+ * If set, only features of the given layers will be returned by the format
+ * when read.
+ * @type {Array.<string>|undefined}
+ * @api
+ */
+olx.format.WMSGetFeatureInfoOptions.prototype.layers;
+
+
 /**
  * Namespace.
  * @type {Object}