oggdec: fix keyframe seeking when granule_is_start is 0.

In this case, the pts values will be delayed by one, but
at the same time pts values might only be supplied for e.g.
keyframes.
This results on only the frame after the keyframe having a
pts value.
As a hack, make read_timestamp return the keyframe position
together with the pts from a following frame when seeking
to a keyframe.
Fixes trac issue #438.
However it causes the read_timestamp function to return a
pos value that is actually before the packet with the
indicated pts.

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