HSL and HSV interpolation for UI gradients

[ickshonpe]

Objective

Add interpolation in HSL and HSV colour spaces for UI gradients.

Solution

Added new variants to InterpolationColorSpace: Hsl, HslLong, Hsv, and HsvLong, along with mix functions to the gradients shader for each of them.

Limitations

• Didn't include increasing and decreasing path support, it's not essential and can be done in a follow up if someone feels like it.

• The colour conversions should really be performed before the colours are sent to the shader but it would need more changes and performance is good enough for now.

Testing

cargo run --example gradients

[alice-i-cecile]

@lynn-lumen, you attempted something very similar in #12939. Would you be open to reviewing this?

[lynn-lumen]

This looks good to me. There are just two functions that I believe should be named differently.

[lynn-lumen]

This is not specific to this PR, but I think the lerp_hue_long function is incorrect. It seems to not take the longest direct path between two colors but rotates 360° before taking the shortest path.

In this image, you can see one gradient with three stops, red, blue and lime at 0%, 50% and 100% interpolated in HsvLong. I would have expected it to look more like this second image.

A similar issue, though less obvious, persists when interpolating in OkLchLong.

[ickshonpe]

A similar issue, though less obvious, persists when interpolating in OkLchLong.

Oh yep good catch, the select cases are the wrong way around, it should be select(diff + TAU, diff - TAU, diff < 0.).

Should be fixed by #20010

[viridia]

It seems to not take the longest direct path between two colors

Actually, I noticed this problem too and forgot to mention it. I ended up breaking the hue rotation into multiple steps, each less than 180 degrees, to work around it.

For the hue slider, I need to go from 0 to 360 (or zero to zero I suppose). To get consistent results, however, I ended up having to go from 0.00001 to 359.9999. This doesn't affect the range of values that the slider can edit, only the background gradient, and the difference is imperceptible.

[viridia]

OK I patched this in to my color sliders PR and can verify that it is working.

One thing I would really like is to be able to import these color functions into a custom shader, but that can be left for a follow-up PR.