Add CellRegion and CellLayer classes to simplify map overlays.
Pass the visible cell region to IRenderOverlay.
Use CellLayers for pathfinder debug.
Use CellLayers for resources.
Use CellLayers for shroud.
Use CellLayer for ActorMap.
Use CellLayer for DomainIndex.
Use CellLayer for custom terrain.
Use CellLayer for resources.
Use CellLayer for terrain.
Remove CellBounds from BuildableTerrainLayer and SmudgeLayer