Function self_intersections_brute_force

pub fn self_intersections_brute_force<M>(mesh: &M) -> Vec<IntersectingPair>
where
    M: TriangleMeshView + ?Sized,

The same answer without the spatial index, by checking every pair.

This exists to be compared against self_intersections. The index is an optimisation, and an optimisation that changes the answer is a bug, so the reference is kept in production code rather than in a test where it could drift out of sync with the accelerated path.