Bug fixed on clone() method

VertexBuffer.references is not defined. VertexBuffer._buffer.references should be used instead.

This fixes a graphical issues when the cloned mesh is being disposed.

Babylon/Mesh/babylon.mesh.js

@@ -836,7 +836,7 @@ var BABYLON = BABYLON || {};
         // Buffers
         result._vertexBuffers = this._vertexBuffers;
         for (var kind in result._vertexBuffers) {
-            result._vertexBuffers[kind].references++;
+            result._vertexBuffers[kind]._buffer.references++;
         }
 
         result._indexBuffer = this._indexBuffer;
@@ -1592,4 +1592,4 @@ var BABYLON = BABYLON || {};
             normals[index * 3 + 2] = normal.z;
         }
     };
-})();
+})();