Use d50 referent instead of d65 for lab (and by extension hcl) color space

[ChrisLoer]

I haven't updated the relevant tests yet, but the ones that are failing look like they're failing in ways that look about right, e.g.:

  ● interpolate › interpolate color › should interpolate colors in "lab" color space

    expect(received).toMatchObject(expected)

    - Expected  - 3
    + Received  + 3

      Object {
        "a": NumberCloseTo 0.9 (4 digits),
    -   "b": NumberCloseTo 0.7461000000000001 (4 digits),
    -   "g": NumberCloseTo 0.32084999999999997 (4 digits),
    -   "r": NumberCloseTo 0.3816 (4 digits),
    +   "b": 0.7501995807994258,
    +   "g": 0.3109760521913497,
    +   "r": 0.3567325378913688,

I also haven't figured out how to get this to properly build and incorporate into a maplibre-gl-js package yet.

cc @ibesora

[ibesora]

What does "x, z;" do here?

[ChrisLoer]

Oops -- nothing, I think. Left over from a different way of having the variables defined earlier.