Fix to GLUT window-close problem

Rob Fletcher, at the University of York, has
written a fix to the GLUT window-close problem.
He’s added a callback which informs the caller when a window is closed. The default action is
to exit, so this is backwards-compatible with existing code. But multi-window GLUT applications can now handle window closes properly. See
http://www-users.york.ac.uk/~rpf1/glut.html