World|Sector: Improved sector cluster mapping to deal with missing texture hacks

This algorithm deals with cases where a physically lower interior
must be mapped visually to a physically higher exterior. Note that
currently this is a static mapping (these cases should actually be
resolved dynamically when a dependent surface moves or loses/gains
a material).

Mapping in the opposite direction (i.e., from a physically higher
interior to a physically lower exterior) will require dynamically
adding an "extra" visual plane to the physical cluster.

Complex constructs such as the yellow key cave in ksutra.wad MAP05
are now interpreted correctly.

Todo: Wall geometry generation needs updating accordingly.