tweak and clean up conversion to perceptual space

- try ITU-R BT.2020 YPbPr conversion instead of ITU-R BT.601 as seems
  to do slightly better on specular highlights plus it's so hip and
  cutting edge
- remove spurious luminance added in when doing chroma differential,
  which was a mystery residual of CIELab
- don't need anything from common/colorspaces.h any more