Up until now, the h264_redundant_pps_bsf stored the initial value
of pic_init_qp_minus26 of the most recently encountered PPS;
it also modified the slices based upon to assumption that
the most recent PPS is the PPS the slice belongs to.
Yet this assumption is flawed, as there can be several PPS
with different IDs that are visible at any given time.
If these have different pic_init_qp_minus26 values,
the output can be invalid.

Fix this by directly using the pic_init_qp_minus26 value of
the input PPS.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>