Multi-color pollution overlay for the map view, scaled to the surface's actual current max chunk
pollution instead of the vanilla fixed 150 cap.

• Toggle on/off with a hotkey (unbound by default - bind it in Settings > Controls > Mods)
• Scanning only runs while the overlay is on and you're looking at the map
• Color gradient adapts to your worst polluted chunk, so the map stays readable in both the early
  and late game