Renderer|LightGrid: Smoothed sector lighting now works with SectorCluster granularity

Rather than working directly with Sectors the bias light grid now
operates with SectorClusters, taking advantage of the extra spatial
knowledge they guarantee/provide.

This has some important implications:
- Degenerate BSP leafs fundamentally cannot influence the lighting
- Z-axis biasing now works with the cluster-mapped plane heights and
is therefore accurate with respect to map hacks

Todo: SectorCluster should now provide the effective ambient lighting
properties within the cluster.