Properly handle negative volume meshes

[elalish]

Discussed in https://github.com/elalish/manifold/discussions/693

^{Originally posted by pca006132 January 11, 2024}

const {cube, sphere} = Manifold;
const foo = cube(3, true).subtract(cube(1, true));
const parts = foo.decompose();
console.log(parts.length);
const result = parts[1];

It would be intuitive for negative volume meshes to be basically equivalent to a union becoming a difference, but we need to figure out if this is compatible with Julian Smith's algorithm.