Merge pull request #890 from tschaub/utfgrid-backbuffer

Override createBackBuffer in UTFGrid layer.

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

tests/Layer/UTFGrid.html

@@ -58,6 +58,21 @@
         layer.destroy();
 
     }
+
+    function test_createBackBuffer(t) {
+      t.plan(1);
+      setUp();
+
+      var got;
+      try {
+        got = layer.createBackBuffer();
+      } catch (e) {
+        got = e;
+      } finally {
+        tearDown();
+      }
+      t.eq(got, undefined, "createBackBuffer returns undefined");
+    }
     
     function test_clone(t) {
         t.plan(3);