Doomsday Engine

Map Renderer: Build triangle strips for BspLeaf sky fix geometry

Rather than write individual quads for sky fix geometry of each edge
of a BspLeaf, form continuous triangle strips to reduce the number
of primitives.

The wall divisions have also been reworked to now hold all Z coords
for each edge of the wall. This affords much simpler logic elsewhere.
Also, the left and right edge divisions are now managed separately.

Wall geometry construction has been reworked to take all Z coords
from the wall divisions.