Corrected a couple of minor issues

Changed city size back to 5x5 (from debugging 1x1), and corrected road
y-osffset so bottoms of road and ground all lined up properly (as
originally intended).
Fixed texture-jumping on overlapping vertical faces

Added code to detect overlapping vertical faces (walls) and to adjust
face vertices so as to eliminate overlaps (and hence, texture flickering
caused by two faces trying to occupy the same location at the same
time).