Wall tiles on edge of map aren't distinct

[Uriah Otting]

When digging through the walls, walls at the far edge of the dungeon are undiggable but look just the same as any other wall. It might be nice to see them as graphically distinct. This could also be a nice enhancement to add in other undiggable walls later.

[Krzysztof Dycha]

Issue #122 was marked as a duplicate of this issue.

[Krzysztof Dycha]

Issue #140 was marked as a duplicate of this issue.