VASliceParameterBufferH264.slice_data_bit_offset will point into the part
of slice header if there are some escaped bytes in the slice header. The offset
to slice data is needed for BSD unit so that BSD unit can fetch right slice data
for processing. This fixes conformance case BASQP1_Sony_C.jsv

The method to get the total number of bytes in the slice header is provided by Gwenole Beauchesne <gbeauchesne@splitted-desktop.com>