Add a central variable g_want_determinism which controls whether to try to make things deterministic.

It now affects the GPU determinism mode as well as some miscellaneous
things that were calling IsNetPlayRunning.  Probably incomplete.

Notably, this can change while paused, if the user starts recording a
movie.  The movie code appears to have been missing locking between
setting g_playMode and doing other things, which probably had a small
chance of causing crashes or even desynced movies; fix that with
PauseAndLock.

The next commit will add a hidden config variable to override GPU
determinism mode.
Add the override config option.

I hate the config code, but now is not the time to fix it...
Test bugfix.