SkyFixEdge: Fully encapsulate edge geometry generation; moved to new source files

The map renderer now interprets an invalid SkyFixEdge to mean there
is no sky fix geometry necessary for the configured wall section.

All sky fix edge geometry generation is now encapsulated within the
SkyFixEdge class (the map renderer itself is no longer concerned with
the sky-fix specific logic for id Tech 1 map-hack interpretation).