mpegts: do not set pts for missing dts in video streams

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: add patch for backport: mpegts-do-not-set-pts-for-missing-dts
Merge pull request #3764 from FernetMenta/mpegts

ffmpeg: backport mpegts fix for dts jumping backwards (trac 14622)