Function cross3

perro_source/render_stack/perro_meshlets/src/lib.rs:693–695

(a: [f32; 3], b: [f32; 3])

    match (current, candidate) {
        (None, x) => x,
        (x, None) => x,
        (Some(a), Some(b)) => {
            if b.cost < a.cost {
                Some(b)
            } else {
                Some(a)