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.
Remove CellBounds from BuildableTerrainLayer and SmudgeLayer
Use CellLayer for custom terrain.
Use CellLayer for resources.
Use CellLayer for terrain.