reordered sponge4 failure

[elalish]

I managed to repro the problem from #171 on master by manually reordering the sponge operations. Many different orders worked fine, but this one, which is most similar to the result of the other PR, did not:

Manifold tmp1 = hole.Rotate(90) + hole.Rotate(90).Rotate(0, 0, 90);
Manifold tmp2 = hole + tmp1;
result = result - tmp2;

Not only did it come up with 39k degenerate triangles, it also ended up with the wrong genus. It's pretty clear there's a problem: