ffmpeg: add video heartbeat capability to fix_sub_duration

Splits the currently handled subtitle at random access point
packets that can be configured to follow a specific output stream.
Currently only subtitle streams which are directly mapped into the
same output in which the heartbeat stream resides are affected.

This way the subtitle - which is known to be shown at this time
can be split and passed to muxer before its full duration is
yet known. This is also a drawback, as this essentially outputs
multiple subtitles from a single input subtitle that continues
over multiple random access points. Thus this feature should not
be utilized in cases where subtitle output latency does not matter.

Co-authored-by: Andrzej Nadachowski <andrzej.nadachowski@24i.com>
Co-authored-by: Bernard Boulay <bernard.boulay@24i.com>

Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>

Changelog

@@ -36,6 +36,7 @@ version <next>:
 - hstack_vaapi, vstack_vaapi and xstack_vaapi filters
 - XMD ADPCM decoder and demuxer
 - media100 to mjpegb bsf
+- ffmpeg CLI new option: -fix_sub_duration_heartbeat
 
 
 version 5.1: