Renderer|WallEdge: Minor cleanup
Renderer|SectorCluster: SectorCluster provides WallEdges for writeAllWallSections()

The owned edges exist over multiple render frames and are currently
allocated on the fly as needed. This is the very first step toward a
more efficient design for map geometry generation.

SectorCluster will guide and/or manipulate its WallEdges dynamically
according to the movement of planes, lighting changes, etc, etc...

At present no WallEdge updating is done whatsoever, meaning that at
present, the wall surfaces remain static regardless of whether their
dependent planes move.

Next step: Update WallEdges when a pertinent change occurs.

Todo:
- Handle special case: edges moving between clusters (polyobjs).
- All WallEdges should be obtained from SectorCluster.