Allow to use core name as save filename

[Kekun]

Add the 'save_filename' variable with the default 'game' value not
changing the current behavior and the 'core' value using a static name
based on the core's filename for every save file.

This allows frontends creating one save directory per game to let the
users rename their game file without loosing their save.

[andres-asm]

This seems overly complicated for no reason.
The current behavior is most likely because it's an unmantained, hardly used core.

I think a better approach would be to follow what other cores do (use libretro srm interface or use the save dir with the game name)

[Kekun]

I thought about allowing access to the save RAM instead of writting save files, but as there are multiple save RAMs here and Libretro don't have RO and WO save RAM accessors like for serialization, but only a RW one you can't serialize/deserialize the save RAM. Instead you would have to put all the save RAMs and their metadata in a contiguous buffer within the emu… It's doable with a LOT more refactoring and it's probably not worth the pain.

This changes is simple and doesn't break the default behavior (hence the current users don't loose their save files) while solving the problem of game file renaming for frontends having one save directory per game and identifying the games with something else than the filename.

[Kekun]

@fr500 As you said the core is unmaintained and hardly used, is there one you would recommend instead? :)

[andres-asm]

mGBA all the way

[inactive123]

We should implement save files through the proper libretro get_memory/set_memory callback functions. It should be discouraged to do save files any other way through a libretro core if we can prevent it.

[negativeExponent]

the core is able to support mednafen save method, as well as using the libretro api. so, this PR might not be relevant anymore.