add support for subtitles embedded in containers (e.g. mkv)

[Petr Nejedly]

these files should be extracted on-the-fly and passed to the device in the same way .srt files are

[Petr Nejedly]

http://forum.doom9.org/archive/index.php/t-148229.html

[Petr Nejedly]

ffmpeg -y -i d:\testclips\bgc.sub.dub.ogm -an -vn -scodec copy -f rawvideo sub.srt

[Petr Nejedly]

Issue #182 was marked as a duplicate of this issue.

[Petr Nejedly]

have to configure when they should be extracted and when the device supports them embedded in the target container (mkv). If mkv is transcoded, we have to extract them separately always.

External subs will have priority over these (for the preferred language).

[Former user]

this does not work for most of my mkv files, ffmpeg shows those subtitle "parts" - but my samsung (d-series) does not show it served via serviio (even if configured in the serviio console to extract them