World|Sector: More stringently classify "missing texture" map hacks

To be eligible for missing texture map hack support, all edges which
define the boundary between sector clusters must not have an exposed
material on the exterior wall section. This extra qualification will
distinguish between cases where the map author has simply forgotten
to assign a texture and where a missing texture hack is intended.

This fixes issues in both Hell Revealed II MAP08 and KSutra MAP05
where mapping errors were misinterpreted as deliberate use of missing
texture hacks.