Make LTN cell calculations account for one-way roads

[dabreegster]

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.

[dabreegster]

Maybe it should be defined per entry into the area? If there's a "loop" like this, if you enter as part of the loop, you're forced out. But if you enter from the other side and join the loop, then it forces you out. Those are different cases. And maybe all the shortcutting traffic is trying to enter from one boundary road, and the other isn't a real problem.