sway: Install the wallpapers by default and use them on NixOS

Size impact:
$ du -sh $(nix-build -A sway-unwrapped)
5.9M    /nix/store/d07zcpybgpdj8xhn9n4qb04lgm453ia1-sway-unwrapped-1.6
nix@sessel:/var/tmp/michael/nixpkgs$ du -sh $(nix-build -A sway-unwrapped)/share/backgrounds
4.9M    /nix/store/d07zcpybgpdj8xhn9n4qb04lgm453ia1-sway-unwrapped-1.6/share/backgrounds

nixos/modules/programs/sway.nix

@@ -119,6 +119,8 @@ in {
     ];
     environment = {
       systemPackages = [ swayPackage ] ++ cfg.extraPackages;
+      # Needed for the default wallpaper:
+      pathsToLink = [ "/share/backgrounds/sway" ];
       etc = {
         "sway/config".source = mkOptionDefault "${swayPackage}/etc/sway/config";
         "sway/config.d/nixos.conf".source = pkgs.writeText "nixos.conf" ''

pkgs/applications/window-managers/sway/default.nix

@@ -20,19 +20,22 @@ stdenv.mkDerivation rec {
   };
 
   patches = [
-    ./sway-config-no-nix-store-references.patch
     ./load-configuration-from-etc.patch
 
     (substituteAll {
       src = ./fix-paths.patch;
       inherit swaybg;
     })
+  ] ++ lib.optionals (!isNixOS) [
+    # References to /nix/store/... will get GC'ed which causes problems when
+    # copying the default configuration:
+    ./sway-config-no-nix-store-references.patch
+  ] ++ lib.optionals isNixOS [
+    # Use /run/current-system/sw/share and /etc instead of /nix/store
+    # references:
+    ./sway-config-nixos-paths.patch
   ];
 
-  postPatch = lib.optionalString isNixOS ''
-    echo -e '\ninclude /etc/sway/config.d/*' >> config.in
-  '';
-
   nativeBuildInputs = [
     meson ninja pkg-config wayland scdoc
   ];
@@ -44,7 +47,6 @@ stdenv.mkDerivation rec {
   ];
 
   mesonFlags = [
-    "-Ddefault-wallpaper=false"
     "-Dsd-bus-provider=libsystemd"
   ];
 

pkgs/applications/window-managers/sway/sway-config-nixos-paths.patch

@@ -0,0 +1,21 @@
+diff --git a/config.in b/config.in
+index 08703bef..f3872730 100644
+--- a/config.in
++++ b/config.in
+@@ -22,8 +22,8 @@ set $menu dmenu_path | dmenu | xargs swaymsg exec --
+ 
+ ### Output configuration
+ #
+-# Default wallpaper (more resolutions are available in @datadir@/backgrounds/sway/)
+-output * bg @datadir@/backgrounds/sway/Sway_Wallpaper_Blue_1920x1080.png fill
++# Default wallpaper (more resolutions are available in /run/current-system/sw/share/backgrounds/sway/)
++output * bg /run/current-system/sw/share/backgrounds/sway/Sway_Wallpaper_Blue_1920x1080.png fill
+ #
+ # Example configuration:
+ #
+@@ -214,4 +214,4 @@ bar {
+     }
+ }
+ 
+-include @sysconfdir@/sway/config.d/*
++include /etc/sway/config.d/*