Apparently we're not supposed to use
`RETRO_ENVIRONMENT_SET_SYSTEM_AV_INFO` to change the FPS. The core
should be running at the display refresh rate and resampling the game's
video output.
This adds a new driver for audio in libretro builds for devices with
multithreading support that defers audio rendering to a worker thread
provided by the libretro frontend.
The threaded driver has the advantage that video lag will not also cause
the audio to lag, which is very noticeable since it manifests in the
form of audio crackling when it happens.
This allows more flexibility when loading games in libretro builds,
since we can now load games either from a directory or from a ZIP or 7Z
archive. Also, the path cache is now active for all filesystem calls
made from inside Ruby.
I've made it so that `Graphics.update` pauses the Ruby VM and returns to
the libretro frontend. Once the libretro frontend calls `retro_run()`
again, the Ruby VM resumes. This allows the libretro frontend to control
the rendering loop.
