nixos/hidpi: add xserver dpi

[aaronjanse]

Motivation for this change

Addresses a TODO comment. More importantly, this should improve NixOS's out-of-the-box experience.

Some additions we may want to consider:

{
  services.xserver.displayManager.sessionCommands = ''
    ${pkgs.xorg.xrdb}/bin/xrdb -merge <<EOF
      Xcursor.theme: Vanilla-DMZ
      Xcursor.size: 32
    EOF
  '';
  environment.variables = {
    GDK_SCALE = "1.5";
    GDK_DPI_SCALE = "0.75";
    QT_AUTO_SCREEN_SCALE_FACTOR = "1";
  };
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[mrkkrp]

FYI, this doesn't really improve NixOS out-of-the-box experience. I just spent 1 hour figuring out why all UI widgets suddenly became oversized on my external screen after an update. The solution? Remove incorrect hardcoded DPI setting:

  services.xserver.dpi = null;

[Artturin]

i'll revert it. and instead add it as a commented line to the configuration.nix that nixos-generate-config creates