Problem: Sometimes one-ways can effectively block off part of an area:

There's just one large purple cell, though, because the current calculation ignores direction of the roads.

This PR is a start to taking them into account:

But even this example gets confusing. Why is there the tiny blue cell at the bottom? There are "spurious" changes in cells based on the order we happen to floodfill. For example we have both:

and

I think this idea has potential, but there are subtleties. A cell is meant to represent everywhere you can reach without leaving the area. One-ways throw that off.