Files are written to the libretro save directory, which is mounted at
/save in PhysFS. All filesystem calls made from Ruby in libretro builds
are routed through PhysFS, so the game can just use any ordinary
filesystem function provided by Ruby to interact with /save.
It's also union mounted on top of the game directory (located at /game
in PhysFS) so that games that write their save files to the current
working directory will have their save files saved to the libretro save
directory instead of the game directory.
For security and portability reasons, nothing outside of the libretro
save directory can be written to, and nothing outside of the libretro
save directory, the libretro game directory and the various embedded
files used by the runtime can be read from.
Any relative paths that the game tries to access in libretro builds will
now be relative to whatever is the current working directory in the Ruby
sandbox, which will also now be initialized to the game directory during
initialization. Before, all of the bindings that took paths were
hardcoded to prepend the path with the game directory.
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.
