mpv: backport ffmpeg-8 support

Without the change `mpv` fails to build on` staging` as:

    ../demux/demux_mkv.c: In function 'demux_mkv_open_sub':
    ../demux/demux_mkv.c:2203:32: error: 'FF_PROFILE_ARIB_PROFILE_A' undeclared (first use in this function); did you mean 'AV_PROFILE_ARIB_PROFILE_A'?
     2203 |                 lav->profile = FF_PROFILE_ARIB_PROFILE_A;
          |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
          |                                AV_PROFILE_ARIB_PROFILE_A

pkgs/applications/video/mpv/default.nix

@@ -9,6 +9,7 @@
   config,
   docutils,
   fetchFromGitHub,
+  fetchpatch,
   ffmpeg,
   freefont_ttf,
   freetype,
@@ -112,6 +113,16 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-x8cDczKIX4+KrvRxZ+72TGlEQHd4Kx7naq0CSoOZGHA=";
   };
 
+  patches = [
+    # ffmpeg-8 compat:
+    #   https://github.com/mpv-player/mpv/pull/16145
+    (fetchpatch {
+      name = "ffmpeg-8.patch";
+      url = "https://github.com/mpv-player/mpv/commit/26b29fba02a2782f68e2906f837d21201fc6f1b9.patch";
+      hash = "sha256-ANNoTtIJBARHbm5IgrE0eEZyzmNhOnbVgve7iqCBzQg=";
+    })
+  ];
+
   postPatch = lib.concatStringsSep "\n" [
     # Don't reference compile time dependencies or create a build outputs cycle
     # between out and dev