UI gradients long hue paths fix

[ickshonpe]

Objective

The false and true arguments for the select statement in lerp_hue_long are misordered, resulting in it taking the wrong hue path:

Solution

Swap the arguments around.

Also fixed another case I found during testing. The hue was interpolated even when it is undefined for one of the endpoints (for example in a gradient from black to yellow). In those cases it shouldn't interpolate, instead it should return the hue of the other end point.

Testing

I added a linear_gradient module to the testbed ui example, run with:

cargo run --example testbed_ui

In the linear gradients screen (press space to switch) it shows a column of red to yellow linear gradients. The last gradient in the column uses the OKLCH long path, which should look like this:

matching the same gradient in CSS:

https://jsfiddle.net/fevshkdy/14/

if the correct hue path is chosen.

[janhohenheim]

Good catch! Code makes sense to me :)

[janhohenheim]

Marking as Waiting-on-Author due to merge conflicts