[demux/avi] Enable dirac support (Set PTS on first output block)
It is possible for a decoder to interpolate all the PTS values at its
output (They are in order and it knows the frame rate). However, an
initial offset is required. Demuxers which only provide DTS (such as
AVI) do not provide the initial offset.
Enable setting PTS=DTS on the first block output from the demux.
NB, this isn't actually correct, it assumes the first PTS=DTS, which
is often not the case. For most MPEG2 style gops this should be ok,
however a sequence that starts with a reordered picture will cause
problems. It isn't trivial to determine the initial offset between
PTS and DTS.
UNTESTED WITH OTHER CODECS