Optimize subtitles support when using embedded + burned-in

[Petr Nejedly]

http://www.ffmpeg.org/ffmpeg-filters.html#subtitles-1

instead of first extracting the subs data and then passing as an external file to the filter we could shortcut it and just use the file subs track in the filter instead.

This should help in cases where subtitles are taking too long to be extracted from mkv files.

[Giovanni Musto]

You can also use filter_complex instead.

[Petr Nejedly]

This should also fix the issue when the str extraction times out but the filter is still being called: http://forum.serviio.org/viewtopic.php?f=8&t=18022&p=97332#p97318