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>
avcodec/vp9/x86: add ff_vp9_loop_filter_[vh]_16_16_ssse3().

16662 decicycles in loop_filter_h_16_16_c, 8387355 runs, 1253 skips
17510 decicycles in loop_filter_v_16_16_c, 8387516 runs, 1092 skips

4941 decicycles in ff_vp9_loop_filter_h_16_16_ssse3, 8387887 runs, 721 skips
3899 decicycles in ff_vp9_loop_filter_v_16_16_ssse3, 8387980 runs, 628 skips

Overall decode time goes from:
  ./ffmpeg -v 0 -nostats -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  8.10s user 0.02s system 99% cpu 8.126 total
to:
  ./ffmpeg -v 0 -nostats -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  6.15s user 0.04s system 99% cpu 6.199 total

(46 to 61 fps)