This isn't exactly semantically equivalent, but the field has already been
long abused to mean this, and writing it helps in determining a decent cfr
time base when transcoding from a mkv where the video codec stores none (VP8).

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23284 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b