Material callback applied

- Crashes on exiting a server or the game, set
$bug::materialsCrashOnExit = 1; before exiting (no memory will leak).
- Not 100% accurate as it turns out.  Sometimes when colliding with ice,
it still says blue.  If you go high up in sky with camera and then drop
marble, marble will report blue on ice, and then on rebound, it reports
correct material. o_O