The split edges code had a complex method of merging duplicate edges,
going backwards to avoid shifting elements in a vector. Sometimes it
could result in incorrect corner edge indices though, if it moved an
index that matched one of the local variables (I think! I've been trying
to understand this all day and still struggling). Instead, replace it
with a `VectorSet` that handles the deduplication by itself, and
avoid creating the new edges until the end of the algorithm.
I think this code could still be simpler if we tried to reduce the
amount of things happening at the same time, making more code
deal with the input or final state rather than an in-between one.
But to avoid making the change too complicated I stopped here.
Hans Goudey