numAliveNeighbors was doing bounds checks too aggressively, resulting in
always returning 8 for points on the edge, meaning walls weren't getting
properly created for those points, making edges of the map open to walk
through.
Change-Id: Iada6be46ce7cc77ce99a320b7310008898b89273
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3805
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI