vaapi_h264: Fix bit offset of slice data.

vaapi_h264: Fix bit offset of slice data.

Commit ca2f19b9cc37be509d85f05c8f902860475905f8 modified the meaning of
H264SliceContext.gb: it is now initialised at the start of the NAL unit
header, rather than at the start of the slice header.  The VAAPI slice
decoder uses the offset after parsing to determine the offset of the
slice data in the bitstream, so with the changed meaning we no longer
need to add the extra byte to account for the NAL unit header because
it is now included directly.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>