POC: vulkan-gaming with HDR using latest gamescope and linux-hdr

[PedroHLC]

I won't merge this PR while all the packages here modified haven't released a new stable version.

But, we can use it as a milestone, or an always up-to-date guide for HDR.

I'm aware ofborg will tag some maintainers, sorry. Please don't hesitate to remove yourself from review and unsubscribe.

This depends on #187507 (included here)

How to use it?

1. Merge this to your nixpkgs or use my branch as nixpkgs source.
2. Add this to your setup:

{ lib, ... }:
{
  # GameScope session
  programs.gamescope.enable = true;
  programs.steam.gamescopeSession.enable = true;

  specialisation.hdr.configuration = {
    system.nixos.tags = [ "hdr" ];
    boot.kernelPackages = lib.mkForce pkgs.linuxPackages_testing_hdr;
    environment.variables =
      {
        DXVK_HDR = "1";
        ENABLE_GAMESCOPE_WSI = "1";
      };
    programs.gamescope.args = lib.mkForce [ "--rt" "--hdr-enabled" ];
  };
}

3. Reboot, picking the new HDR entry in your bootloader.
4. Select the new “Steam Gamescope” session, or, login in tty3 and launch steam-gamescope.
5. Your display should change to HDR mode automatically, Steam colors should be fine.
6. Search in your steam library for the "Steam Linux Runtime - Soldier" (should be under "Tools") and change in "Properties" > "Beta" for opt into "client_beta".
7. Go game something, use recent Proton versions (experimental or GE)!

[PedroHLC]

Thanks to @Scrumplex with #219548 we have most of what was needed here in master now, this time using tagged versions + patches.

[PedroHLC]

Most of this is already merged upstream now, except linux_hdr which will be unstable until upstreamed and does not comply with nixpkgs. If anyone ever needs it, I keep it updated and cached in this project: https://github.com/chaotic-cx/nyx