This avoids some issues with H.264 PAFF due to dropping PTS values too
early because only every second packet actually produced output.
Just keeping up to one additional pts value would have avoided this
particular issue as well, but this is more generic.
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@31312 b3059339-0415-0410-9bf9-f77b7e298cf2