Add Squircle primitive

[lynn-lumen]

Objective

• Adds a squircle primitive, Fixes Squircle / Superellipse for UI / Gizmos / Mesh #13469

Solution

• Added a Squircle primitive to bevy_math. The chosen squircle is based on the superellipse.
• Implemented gizmos for squircles
  • squircles use rectangles and rhombuses internally if the shapes are sufficiently similar.
  • The resolution of the squircle can be configured by calling .resolution() on the builder

Additional information

The gizmos implementation may seem overcomplicated, but the following images may clarify why an implementation like this is needed:

• The left squircles use samples that are equally spaced by angle.
• The center squircles use samples interpolated between the left and right (this is the solution I ended up using)
• The right squircles use samples that have equally spaced normal vectors, i.e. the normal vector rotates by the same amount between each sampled point.
• All squircles have a resolution of 32

[alice-i-cecile]

Docs are great, I quite like the code implementation, and I agree with your choice of gizmo drawing technique, cursed as it is.

My one concern centers around utility. This is a UI-centric addition, but there's no way to use this in UIs currently, and it's not clear how users would do so. Are there other game dev uses to consider? Should we merge this anyways and tackle adding rounded corner support using squircles in a follow-up?

[lynn-lumen]

Should we merge this anyways and tackle adding rounded corner support using squircles in a follow-up?

Yeah, I would prefer that to be in a follow up, since that would be a completely new solution touching at least one more crate

[alice-i-cecile]

@lynn-lumen looks like this needs some love with the recent changes on main.

[rparrett]

Is there a variation of this type of shape that would have "symmetrical curvature" even when width and height are not equal?

To crudely illustrate: https://www.desmos.com/calculator/r4mzfslt8m

As-is, this seems less useful than it could be.

[alice-i-cecile]

Bumping to controversial: I'm not actually sold on how useful this is. Maybe the UI designers know something I don't, but this doesn't seem to be a meaningful improvement over rounded corners.

[lynn-lumen]

There seems to be an issue with access_kit_macos which prohibits building bevy. I have filed an issue #14956