Introduce new GstVaapiDecoderUnitH264 object, which holds the standard
NAL unit header (GstH264NalUnit) and additional parsed header info.
Besides, we now parse headers as early as in the _parse() function so
that to avoid un-necessary creation of sub-buffers in _decode() for
NAL units that are not slices.

This is a performance win by ~+1.1% only.