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.
Is it possible to make an accessor function for this? Only suggesting it as a way to avoid exposing a variable to be global.

If that's more work that is should be though, feel free to ignore this.