ffmpeg: remuxer: fix "initialization discards 'const' qualifier from pointer target type" (#80)

* ffmpeg: fix compilation on `bookworm`
* workflows: test on `bookworm`

Co-authored-by: FacuM <facumo.fm@gmail.com>

util/ffmpeg/remuxer.c

@@ -94,7 +94,12 @@ int ffmpeg_remuxer_open(ffmpeg_remuxer_t *remuxer)
   if (remuxer->packet)
     return 0;
 
-  AVInputFormat *input_format = av_find_input_format(remuxer->input_format);
+#if LIBAVFORMAT_VERSION_MAJOR < 59
+  AVInputFormat *input_format;
+#else
+  const AVInputFormat *input_format;
+#endif
+  input_format = av_find_input_format(remuxer->input_format);
   if (!input_format)
     return AVERROR(EINVAL);