Check flatpak runtime dir first on Linux

[0chroma]

This change would allow SlimeVR to work when the server is running inside a flatpak.

This change has SlimeVR look for sockets in $XDG_RUNTIME_DIR/app/dev.slimevr.SlimeVR before other locations, since this is the only place inside $XDG_RUNTIME_DIR SlimeVR-Server can write to when running inside a flatpak.

If Steam is also running inside a flatpak, we can gain access to this folder on the host by running flatpak override --user --filesystem=xdg-run/app/dev.slimevr.SlimeVR:create com.valvesoftware.Steam.

[ImUrX]

SteamVR beta has sandboxed $XDG_RUNTIME_DIR, I don't think we should keep using it

[0chroma]

I see. If you'd like, I can modify it to look in ~/.var/app/$FLATPAK_ID instead.

That's too bad since the runtime dir is technically where it belongs

[ImUrX]

i guess the current idea is to go to XDG_DATA_DIRS instead

[ImUrX]

I see. If you'd like, I can modify it to look in ~/.var/app/$FLATPAK_ID instead.

That's too bad since the runtime dir is technically where it belongs

but you just set your launch argument to this and it magically fixes steamvr:
~/.local/share/Steam/steamapps/common/SteamVR/bin/vrmonitor.sh %command%

it can see $XDG_RUNTIME_DIR if you do this