GPU determinism (ok, not really ready for merge)

Please see the commit messages for what this does.

This mode (because it's a hack) now turns on and off automatically with netplay/movie recording, with a hidden config option to override it for testing.

Actually, I need one more commit... but I would like review.
It should be off by default, not off no matter what? I guess what we want is it to be off for GPU_DETERMINISM_AUTO for movies, but on for GPU_DETERMINISM_FAKE_COMPLETION ? Right?