oggenc: fix condition when not to flush due to keyframe granule.

oggenc: fix condition when not to flush due to keyframe granule.

The previous condition of 0 page size was wrong,
that would disable the mechanism for all frames at
a start of a page, thus some keyframes still would not
get their own granule.
The real problem is that header packets must not be flushed,
but they have (and must have) 0 granule and thus would
be detected as keyframes.
Add a separate parameter to mark header packets.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>