flatpak: Bundle a Mega Drive/Genesis Libretro core
Migrated from https://bugzilla.gnome.org/show_bug.cgi?id=772082
We should provide a functional Libretro core for playing Mega Drive/Genesis games.
BlastEm seems to be the best candidate as it is free software, unfortunately the version ported to Libretro is old and very rough around the edges.
A port of a more recent and good enough version is in progress: https://github.com/libretro/blastem-libretro/issues/4