Map touchup: Corinthian isthmus

• Support biomes
• Hills, forests, food & metal mines won't spawn on the isthmus anymore, to avoid obstructions and make things a little fairer
• Stone mines spawn only on the isthmus and players start only with small stones
• Tidier forests
• The central passageway is sometimes slightly offset left/right
• The central passageway is surrounded by a walkable shallow, which prevents effectively walling the actual passageway.
• Widen the river slightly.

Overall the intention is to make the map less of a turtle-fest, though it remains chokepointy by nature.

Differential Revision: https://code.wildfiregames.com/D4062