* Split replay.c into multiple files under replay/; improve logical
separation of replay-related code.
* Separate replay playback state from data.
* Get rid of global static replay struct and avoid unnecessary replay
copying.
* Replay playback and recording are now independent and may occur
simultaneously, although this functionality is not yet exposed. This
enables replay "re-recording" while synthesizing new desync check
events, possibly at a different rate from the original replay.
* Rate of recorded desync check events can now be controlled with the
TAISEI_REPLAY_DESYNC_CHECK_FREQUENCY environment variable. The default
value is 300 as before.
* Probably other stuff I forgot about.
