change SpriteBatchNode to only create renderTexture when it will be used

When profiling for mobile web, we found that CCLabelBMFont was very slow to create (in the order of 20-50 ms per instance).  Some digging revealed that most of that time was spent creating and resizing the renderTexture.

As that texture is only actually used by CCTMXLayer, I switched around the code to only create the renderTexture when it will actually be used.