cutTwin method
Lets go of whatever was on the other side of halfEdge, leaving it on a
boundary.
What an operation calls when it takes a piece of the surface away from
its neighbours without moving it — splitSelection — and the one write
that turns a shared edge into two rims.
Implementation
void cutTwin(int halfEdge) {
final twin = _twin[halfEdge];
if (twin == none) return;
_wrote = true;
_twin.write(twin, none);
_twin.write(halfEdge, none);
}