Refactor|Renderer|SkyFixEdge|WallEdge: Revised WallEdge and SkyFixEdge to prepare for merge

WallEdge and SkyFixEdge now manage surface sectioning internally.
This presently means that all sections for an edge are prepared for
use each time, regardless if only one of them is used. However, the
edges will soon be reworked into SectorCluster-internal mechanisms
and will exist over multiple frames.

Todo:
- Merge WallEdge and SkyFixEdge into one
- Give SectorCluster ownership of edges and provide a public API to
access them in terms of logical geometries
- Cleanup