mpegts muxer, DVB subtitles encoder: common DVB subtitles payload

Improved DVB subtitles encoder to generate AVPacket.data in the same
format as generates MPEGTS demuxer + DVB subtitles parser. So now single
format of DVB subtitles data is used across all the components of FFmpeg:
only subtitles payload WITHOUT 0x20 0x00 bytes at the beginning and 0xFF
trailing byte.

Improved MPEGTS muxer to support format of DVB subtitles in
AVPacket.data described above: while muxing we add two bytes 0x20 0x00 to
the beginning of and 0xFF to the end of DVB subtitles payload.

The patch fixes DVB subtitle copy problems: tickets #2989 fully and #2024
partly.

Signed-off-by: Clément Bœsch <u@pkh.me>