API: h264 decode: fix the definition of slice_data_bit_offset.

API: h264 decode: fix the definition of slice_data_bit_offset.

Align with the existing practice in various media stacks available
in Linux today. i.e. codec layer submits slice data that represents
the original bitstream, thus including any emulation prevent bytes.
However, the slice_data_bit_offset represents an offset relative
to the buffer that contains the NAL unit byte and the rest of the
slice with any emulation prevention bytes removed.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>