sway: Scale wallpaper with waifu2x-converter-cpp

This has been disabled since 2e33f18a3a,
but since I now have a 4k screen it is useful again.

users/simon/modules/sway/default.nix

@@ -1,15 +1,22 @@
 { config, lib, nixosConfig, pkgs, ... }:
 let
   # Source: “境目” by 3211 on Pixiv: https://www.pixiv.net/en/artworks/39266182
-  wallpaperUnfree = pkgs.fetchurl {
-    name = "wallpaper-unfree";
-
+  wallpaperUnfree = pkgs.runCommand
+    "wallpaper-unfree"
+    {
+      meta.license = lib.licenses.unfree;
+    }
+    ''
+      ${pkgs.waifu2x-converter-cpp}/bin/waifu2x-converter-cpp \
+          -i ${pkgs.fetchurl {
             url = "https://i.pximg.net/img-original/img/2013/10/22/04/25/37/39266182_p0.jpg";
             sha256 = "053gc9jd4cbkkwgcirrhpzbn933dfh83l30p1sz55m5d8zx1lk65";
             curlOpts = "--referer https://pixiv.net";
-
-    meta.license = lib.licenses.unfree;
-  };
+          }} \
+          -o out.png
+          # -o $out does not work
+          cp out.png $out
+    '';
   wallpaperFree = pkgs.fetchurl {
     url = "https://raw.githubusercontent.com/swaywm/sway/3b2bc894a5ebbcbbd6707d45a25d171779c2e874/assets/Sway_Wallpaper_Blue_1920x1080.png";
     sha256 = "1rkqd0h7w64plibn7k3krk5vdc3pnv3fc7m2xc2mxnwrbsgngwsz";