This typically happens when the window is being closed. As a general rule, in
ReleaseGraphicsResources() check to make sure that the window exists and is
"Mapped" before making any OpenGL calls. Otherwise simply assume that the
context was destroyed so the OpenGL objects were released anyways, and just
update internal ivars to reflect the same.

This should hopefully fix all the bizzare segfaults on old mac machines and
windows machines.