Map Renderer|Sector|WallEdge: Prevent potential t-join artifacts on walls near mapped planes

WallEdge now uses the half-edge rings and the mapped planes of sector
clusters when adding neighbor intercepts/divisions.

Added SectorClusterCirculator to simplify/clean up traversals of the
cluster's half-edge boundary. Made use of the circulator in WallEdge
and SectorCluster itself (when initializing the boundary info).