Editing one-way walls very buggy, especially Undo and Mark/Clone

[rversteegen]

One-way walls are implemented using zones instead of passmap bits, which it turns out was a big mistake. A lot of complexity and many bugs and missing features in the map editor have resulted. They're all interrelated, so I'll document them all here:

• When placed by pressing W, they aren't tracked by undo/redo!

• If Clone Tile Merging is Off, then the Clone preview cursor converts tiles with one-way wall bits set into all wall bits set (255). This happens when the Clone preview is moved over that tile. Note that the preview is implemented using an undo/redo buffer.

• If Clone Tile Merging is On, one-way walls aren't destroyed, but they aren't copied as they should be. If you play around with Mark/Clone in the Zonemap mode you see the same thing.

• Foxley reported all wall bits can also be set when trying to undo/redo placing a one-way wall: https://cdn.discordapp.com/attachments/275093196852166667/387127463572144129/action370001.gif "I actually never set any of those bits, was just messing around with Ctrl+arrow keys and W for one way walls"

• Pressing W while using the clone tool results in all wall bits (255) being written to the tile under the cursor, although you can't see it because the clone preview hides it

• There are lots of ways in which one-way walls don't work properly when using the other tools, for example if you use the Box tool and press Space is fills the box with walls, if you press W it doesn't.

• They aren't reported/handled by the wallmap script commands, you have to use zone commands instead

[rversteegen]

I've fixed the problems related to undo, mark and clone (r11316) but not the non-standard behaviour of other tools, which will be easiest to fix by turning one-way walls into wall bits, even if they aren't actually stored on disk that way, a write/readblock wrapper could treat it as if it was a wall bit.