[fix] display of walls in sliced undeground view

[Dwachs]

In sliced underground view there are two problems now:
(1) walls facing back are not displayed
(2) walls are drawn on top of tunnel mouth if the tunnel is at the slice level.

Patch attached. I do not knwo whether it breaks something else. Please test.

[kierongreen]

Confirmed bugs however patch doesn't work correctly with double heights. Will investigate more tomorrow if I get a chance.

[Dwachs]

Committed a better patch, see r 6807. It should also work properly with double height pak (tested with pak128.Britain).

[kierongreen]

Great thanks