Add lightColor to Cesium3dTilesTraits

[ykiu]

What this PR does

Expose the lightColor setting for 3D Tilesets in Cesium3dTilesCatalogItem.

Test me

Start a local dev server on localhost:3001 and compare two lightColor settings.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[na9da]

Hi @ykiu,

Thanks for contributing the change.
I have left a suggestion.

Cheers

[na9da]

Suggestion: Most other color traits in Terria are typed as a string and then converted to Cesium.Color using Cesium.Color.fromCSSString() (example). I think we should follow the same pattern here. What do you think?

[ykiu]

Thanks for your suggestion,@na9da. That's what I initially went with, but lightColor doesn't seem to be a standard 24-bit RGB color. When I compare [4095, 4095, 4095] and [255, 255, 255], the former appears brighter. So, I assume limiting it to the [0-255] range is not relevant here.

Left: [255, 255, 255]; right: [4095, 4095, 4095]