[ffmpeg] Set default subtitles regardless of container

yt_dlp/postprocessor/ffmpeg.py

@@ -639,6 +639,8 @@ def run(self, info):
             # Don't copy the existing subtitles, we may be running the
             # postprocessor a second time
             '-map', '-0:s',
+            # Mark the first subtitle track as default
+            '-disposition:s:0', 'default',
         ]
         for i, (lang, name) in enumerate(zip(sub_langs, sub_names)):
             opts.extend(['-map', '%d:0' % (i + 1)])