* we have a Dither2Yuv base class. Currently this is not derived
  from a basic ditherWrapper class because we don not have this
  in mpeglib yet.
  TODO: change in mpeglib DitherWrapper->Dither2RGB and
        make DitherWrapper pure virtual and derive Dither2YUV
        Dither2RGB from this class.

* Note we do not support 8 Bit here, thus the constructor looks
  dofferent.