All the webm/vp9 files I have seen so far can have packets that contain
1 invisible and 1 visible frame. The vp9 parser separates them. Since
the invisible frame is always (?) the first sub-packet, the new packet
is assigned the PTS of the original packet, while the packet containing
the visible frame has no PTS.

This patch essentially reassigns the PTS from the invisible to the
visible frame.
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>