Use d50 white point instead of d65 in HCL/LAB color spaces

[ChrisLoer]

See: maplibre/maplibre-gl-js#2382

The RGB -> HCL code in https://github.com/maplibre/maplibre-gl-js/blob/main/src/style-spec/util/color_spaces.ts forked out of d3-color in 2016. It uses the D65 illuminant in all of its intermediate color spaces. In 2018 d3-color was modified to convert from D65 to D50 (see discussion at d3/d3-color#42). The CSS spec recommends using D50 to convert to LAB (which is the step before converting to HCL).

This PR updates the code using constants derived from the modern d3-color (:wave: thank you @mbostock!), with a link to the helpful explanation at https://observablehq.com/@mbostock/lab-and-rgb.

@kajkal put together a very helpful demo showing the effect of this change (along with many other recent changes they've been working on): https://kajkal.github.io/maplibre-gl-style-spec-color-demo/swipe.html. By swiping up and down, you can see the d65 vs d50. The difference is subtle, but if you look at the "blue->white" section of the "interpolate-hcl" line, you can see that with d65 the whole range of colors looks whiter than with d50.

This does change existing behavior, but in a pretty subtle way, and no APIs/documentation etc. need to change as far as I'm aware. As @kajkal points out, there were much bigger problems (and fixes) in color handling that will go into the 3.x maplibre-gl release, and I hope this will just be an extra bit of tidying up.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[ChrisLoer]

Oops, there was one linter change that failed in CI -- should be fixed now

[HarelM]

Thanks!!

[ChrisLoer]

@HarelM now that this is merged, how should we move it towards release? Is the plan something like:

• Merge Migrate expression tests #97 to migrate expression tests into maplibre-style-spec
• Do a 19.1.0 release (or 19.0.2, might have to read the semver spec...)
• Make a PR to maplibre-gl-js that simultaneously bumps the package version, migrates the tests, and picks up the new behavior?

We could alternatively try to push this ahead of #97 but it seems like it would add a bunch of extra release/update/migrate steps. cc @birkskyum

[HarelM]

Release a new npm package in this repo and bump it in maplibre-gl.
If you have a few minutes to push this forward you can create a PR with the version bump and changelog changes.
See #134 for example.