Minimap rendering does not take unknown tile specs into account

[ethanmoffat]

Unknown TileSpec (e.g. for fishing on BU) should be rendered based on the vanilla client. This hasn't been tested.

Need to create a map with a range of unknown tile specs and see what the behavior is on the vanilla client. Either all tile specs should be rendered as visible blue exclamation point or there are different rules based on value ranges.