Map Renderer: Began consolidating sky-fix geometry generation

Began reworking this logic with the same edge-centric model as used
by the SectionEdge and ShadowEdge classes.

Wrapped up the sky-fix triangle strip generation into a preliminary
TriangleStripBuilder class. Presently strips can only be built by
adding new edges.

Added C++ ctors to rvertex_t and rtexcoord_t for cleanliness.