swaybg: add webp support

pkgs/by-name/sw/swaybg/package.nix

@@ -10,6 +10,8 @@
   wayland-protocols,
   cairo,
   gdk-pixbuf,
+  gnome,
+  webp-pixbuf-loader,
   wayland-scanner,
   wrapGAppsNoGuiHook,
   librsvg,
@@ -50,6 +52,18 @@ stdenv.mkDerivation rec {
     "-Dman-pages=enabled"
   ];
 
+  # add support for webp
+  postInstall = ''
+    export GDK_PIXBUF_MODULE_FILE="${
+      gnome._gdkPixbufCacheBuilder_DO_NOT_USE {
+        extraLoaders = [
+          librsvg
+          webp-pixbuf-loader
+        ];
+      }
+    }"
+  '';
+
   meta = with lib; {
     description = "Wallpaper tool for Wayland compositors";
     inherit (src.meta) homepage;