Flatpak udev permissions

udev permissions in flatpak are a known issue. Right now, we're using SDL and x as a stop-gap:

https://github.com/flathub/org.libretro.RetroArch/blob/master/retroarch.cfg#L28-L29
``` ini
# Input driver. Depending on video driver, it might force a different input driver.
input_driver = "x"

# Joypad driver. ("udev", "linuxraw", "paraport", "sdl2", "hid", "dinput")
input_joypad_driver = "sdl2"
```

This is to track that change and see what alternatives we could take on.

Start with retroarch.cfg at the linked lines and review the current SDL2 joypad and X input-driver workaround. Investigate what alternatives are available for udev permissions in Flatpak. Done would mean identifying and agreeing on a viable replacement or documenting why the workaround must remain.

Written by the indexing model from the issue text.