libretro port

[eadmaster]

Would add support for a lot more systems.

[vgmoose]

I'm interested! Looks like it shouldn't be too hard: https://docs.libretro.com/development/cores/developing-cores/

Probably the sanest thing to do is to add a new libretro/non-SDL framebuffer drawing backend to Chesto. (A goal partially shared by the SDL1 platforms (3ds+wii) too, which are lacking an SDL2 fontcache-equivalent)