Avoid redundant cell <-> map conversions

The CellRegion and CellLayer classes prefer map-coords internally since they can be used more directly for array indexing, but their APIs all expose cell co-ordinates.

The lead to a particularly bad case where enumerating the cells of a region would do map->cell converions for you, only to use those cells as an indexer into a layer which would convert the cell back into a map co-ordinate. All the round trips are quite expensive since so many are performed.

This PR adds the ability to enumerate a region in map-coords and then uses this ability in several key places to index into the layer using those map-coords. I didn't fix every use since there's so many.
This is just listing the exisiting conversion out of the map.Contains method. The new IsVisible(int u, in v) method lets anything already dealing in map coordinates skip a redundant conversion.