nal->skipped_bytes_pos contains the positions of errors relative to the
start of the slice header, whereas the position they were tested against
is relative to the start of the slice data, i.e. one byte after the end
of the slice header.

Patch fixes this by storing the size of the slice header in H266RawSlice
and adding it to the position given by the GetBitContext before
comparing to skipped_bytes_pos.  This fixes AVERROR_INVALIDDATAs for
various valid bitstreams, such as the LMCS_B_Dolby_2 conformance
bitstream.
Signed-off-by: Frank Plowman <post@frankplowman.com>