flatpak: Use the BlastEm core from Libretro
It should be in a good enough shape to be used now.
Fixes https://gitlab.gnome.org/GNOME/gnome-games/-/issues/74
Draft as we still have to fix that snapshot issue:
Capture_d_écran_vidéo_de_11-12-2020_13_43_15
CC @ailepet as you were interested by this, don't hesitate to try it. :)