Added render callback feature to graphics output objects.  This is very similar to the CallbackGraphicsWindow already built in to Panda, but it offers this feature at a lower level.  Adding this feature at this level enables users to have Panda do the setup work for a graphics context and still hook in to the render loop for custom processing.  The CallbackGraphicsWindow requires users to setup their own graphics devices as well as keyboard and mouse input and thus aren't as convenient.