Render callbacks

[Geoffm]

I have extra user-data on each object, and this needs to be interpreted once per redraw of the scene (all 4 windows will show same appearance).

If the update reported its window then I could detect the first redrawn window. However if only one view is on screen I only get 2 callback events and the window which updates the user-data might not be used (so I wont get the update).

Geoff