Point of Interest overdraw can create overlapping buildings

This is really annoying when it happens to important POIs like geysers.

One possible solution would be to add a bool to each cell in POIs, which can be used to mark important/non-overwriteable cells. Add a list of bools to the grid (one bool for each cell) and update it when placing POIs. When placing, check that important cells in the new POI doesn't conflict with cells already set in the grid list. The grid list can be discarded once all POIs are placed and it could be a temporal list, which isn't stored in Grid itself.

If implemented like this, one possible good addon would be to avoid important cells from being overwritten by non-POI stuff like map edges.