Run cores into a subprocess
Libretro cores should be run into a subprocess, this would ultimately allow to:
- simplify identifying the calling-back core, hence
- support cores calling back from a worker thread,
- make retro-gtk more resilient to crashes in the Libretro core,
- optionally use libseccomp to mitigate attacks coming from malicious cores or malicious games.
To do so, I propose to split RetroCore into a stub living in the user's process and a skeleton living in a worker process.