imv: add patch for libnsgif (#388255)

pkgs/by-name/im/imv/package.nix

···
   stdenv,
   lib,
   fetchFromSourcehut,
+  fetchpatch,
   asciidoc,
   cmocka,
   docbook_xsl,
···
   icu75,
   pango,
   inih,
-  withWindowSystem ? null,
+  withWindowSystem ? if stdenv.hostPlatform.isLinux then "all" else "x11",
   xorg,
   libxkbcommon,
   libGLU,
   wayland,
-  # "libnsgif" is disabled until https://todo.sr.ht/~exec64/imv/55 is solved
   withBackends ? [
     "libjxl"
     "libtiff"
···
     "libpng"
     "librsvg"
     "libheif"
+    "libnsgif"
   ],
   freeimage,
   libtiff,
···
 }:
 
 let
-  # default value of withWindowSystem
-  withWindowSystem' =
-    if withWindowSystem != null then
-      withWindowSystem
-    else if stdenv.hostPlatform.isLinux then
-      "all"
-    else
-      "x11";
-
   windowSystems = {
     all = windowSystems.x11 ++ windowSystems.wayland;
     x11 = [
···
 in
 
 # check that given window system is valid
-assert lib.assertOneOf "withWindowSystem" withWindowSystem' (builtins.attrNames windowSystems);
+assert lib.assertOneOf "withWindowSystem" withWindowSystem (builtins.attrNames windowSystems);
 # check that every given backend is valid
 assert builtins.all (
   b: lib.assertOneOf "each backend" b (builtins.attrNames backends)
···
   };
 
   mesonFlags = [
-    "-Dwindows=${withWindowSystem'}"
+    "-Dwindows=${withWindowSystem}"
     "-Dtest=enabled"
     "-Dman=enabled"
   ] ++ backendFlags;
···
       pango
       inih
     ]
-    ++ windowSystems."${withWindowSystem'}"
+    ++ windowSystems."${withWindowSystem}"
     ++ builtins.map (b: backends."${b}") withBackends;
 
+  patches = [
+    (fetchpatch {
+      # https://lists.sr.ht/~exec64/imv-devel/patches/55937
+      name = "update libnsgif backend";
+      url = "https://lists.sr.ht/~exec64/imv-devel/[e-mail]%3E/raw";
+      hash = "sha256-/OQeDfIkPtJIIZwL8jYVRy0B7LSBi9/NvAdPoDm851k=";
+    })
+  ];
+
   postInstall = ''
     install -Dm644 ../files/imv.desktop $out/share/applications/
   '';
 
-  postFixup = lib.optionalString (withWindowSystem' == "all") ''
+  postFixup = lib.optionalString (withWindowSystem == "all") ''
     # The `bin/imv` script assumes imv-wayland or imv-x11 in PATH,
     # so we have to fix those to the binaries we installed into the /nix/store