Bug fixes related to OpenGL.

1) Model parts didn't save the display list generated. (you could cause a 'post render error' by changing to third person mode)

2) Display lists are now properly cleaned up. This saves large amounts of memory when switching to a smaller world. (you can test this by generating a huge world in single-player, then a small world, and comparing the memory usage of the process when the small world has been generated before and after this commit)