I still don't fully understand the cause but the difference between
the samples that trigger the bug and the samples that don't is
that the former uses delay frames and the later uses drop frames
as placeholders for the packed frame. So, if we see the one type
of frame, we can assume the bug will or won't be present.

Right now, I'm detecting the frame types by size, which may not be
safe in general, but given the specific codec and file type, I
expect any scenario where we encounter these frames where they
aren't being used for b-frame packing won't care one way or
another whether the work around is in effect or not.
Signed-off-by: Philip Langdale <philipl@overt.org>