pacakging for flatpak

[mio-19]

Describe the solution you'd like
add flatpak packaging for various linux distros

Additional context
SR2 did have a flatpak package

[cryham]

There were already efforts on making SR 3.2 flatpak,
discussed here: flathub/org.tuxfamily.StuntRally#15 and flathub/org.tuxfamily.StuntRally#17
Failed most likely because of my fork of MyGui not being fully CMake compatible, install command wasn't made as required by that build system. So it only woked using my script in local folders. MyGui didn't install into system folders.
I think if someone fixed that issue then building SR3 flatpak could resume.
Like I already mentioned I don't have that CMake knowledge or time for SR anymore.

[mio-19]

nix is an alternative if goal is to run on different linux distro

https://nixos.org/download/

https://github.com/mio-19/nurpkgs/tree/main/pkgs/stuntrally3

https://github.com/nix-community/nixGL