Packets come from 1394 480 bytes at a time.  This is not a video segment
length.  This causes problems, since a packet boundary crossing a video
segment can split a video segment if we lose an iso packet.  We can
recover from this, sorta, with significant changes to the parser.  We have
to deal with the idea that a) some macroblocks just don't exist (we have
zero's for them) and b) when any of the 5 macroblocks doesn't exist, we
can't do pass 3.

Since things are bitstream-based, we can deal with this, but we have to
add a layer of code that tries to save time (maybe) by not decoding things
that don't exist.  Not sure how this is gonna work with the parse code
being based on video segments, and not easily splittable into
macroblock-level parsing (or is it?).
