Grant access to Gamescope socket

[rakonkido]

I encounter this error every time I launch a RetroArch game through Steam Deck's Game Mode UI:

I've encountered it many times before when launching games through other Flatpaks, like those for Lutris, Heroic, and other emulators. Recently, I found that this error went away after adding support for HDR on Steam Deck to Lutris Flatpak -- itself based on the method used for enabling HDR support on Heroic and Chiaki4deck.

Similar to PRs I have made for Dolphin, PrimeHack, and PPSSPP, I have tested and confirmed that just adding the gamescope-0 socket is enough to make the error go away, with no apparent regressions, if you have the Gamescope Flatpak installed:

flatpak install flathub org.freedesktop.Platform.VulkanLayer.gamescope

[flathubbot]

Started test build 94804

[flathubbot]

Build 94804 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/77552/org.libretro.RetroArch.flatpakref

[RobLoach]

Thanks a lot for testing on here.