Overriding createBackBuffer in UTFGrid layer

This layer doesn't need to create a backbuffer.

lib/OpenLayers/Layer/UTFGrid.js

@@ -109,6 +109,12 @@ OpenLayers.Layer.UTFGrid = OpenLayers.Class(OpenLayers.Layer.XYZ, {
             utfgridResolution: this.utfgridResolution
         }, this.tileOptions);
     },
+
+    /**
+     * Method: createBackBuffer
+     * The UTFGrid cannot create a back buffer, so this method is overriden.
+     */
+    createBackBuffer: function() {},
     
     /**
      * APIMethod: clone