WIP: fix(easyNvidia?): Make prime offload work on sway

Turns out that:

- The usual env variables make everything use nvidia even if it
  shouldn't (duh)
- To use a non-default GPU on wlroots, use `$WLR_DRM_DEVICES`
  - On my current testbed the default GPU happens to be nvidia, see
    sway docs on how to handle this elsewhere
- Using unstable nixpkgs in nixpkgs-wayland can cause issues with
  drivers

Still need to confirm if the addition of `"amdgpu"` to the
`videoDrivers` list is actually necessary, and figure out how to
ensure that powermanagement is actually turned on, see #383.

*Then* set up a good offload script, also see #383 for hints.

flake.nix

@@ -17,7 +17,10 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
-    nixpkgs-wayland.url = "github:nix-community/nixpkgs-wayland";
+    nixpkgs-wayland = {
+      url = "github:nix-community/nixpkgs-wayland";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
 
     nix-gaming = {
       url = "github:fufexan/nix-gaming";

nixos-config/hosts/rin/default.nix

@@ -40,6 +40,16 @@
     amdgpuBusId = "PCI:5:0:0";
   };
 
+  services.xserver.videoDrivers = [ "amdgpu" ];
+
+  environment.variables = {
+    # For wlroots, order the AMD card *before* the NVIDIA card.
+    #
+    # In practice, this assignment is probably not static; see the
+    # sway wiki for some udev hacks to resolve this.
+    WLR_DRM_DEVICES = "/dev/dri/card2:/dev/dri/card1";
+  };
+
   networking = {
     hostName = "rin";
     hostId = "e6aaf496";

nixos-modules/nvidia/default.nix

@@ -71,7 +71,7 @@ in
         ];
     };
 
-    environment.variables = {
+    environment.variables = lib.mkIf (!cfg.offload.enable) {
       # Required to run the correct GBM backend for nvidia GPUs on wayland
       GBM_BACKEND = "nvidia-drm";
       # Apparently, without this nouveau may attempt to be used instead