CornerRadius border radius value semantic and ergonomic improvements

[ickshonpe]

Objective

The elliptical border-radius PR was merged with a slightly half baked API.

In particular, a corner given a single radius value should be circular or square. For example, with:

Node {
    width: px(200),
    height: px(100),
    border_radius: px(100),
}

it should clamp the radius on both axes to the min half size (50) so we get circular corners, but on main atm you get an elliptical (100, 50) radius corner.

Solution

• Val2 has the wrong semantics. Added a new struct CornerRadius { x: Val, y: Val }.
• A CornerRadius created from a single val: Val is mapped to CornerRadius { x: val, y: Val::Auto }.
• If either field is zero or both are auto, the corner will be square.
• If one field is auto, the resolved radius will be circular with the radius clamped to max half of the min of the node's width or height.

Still needs a bit more bikeshedding and some additional docs.

Testing

There is a new test and an extra example case in the elliptical radius UI testbed.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-25050

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-25050

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.