Cache tunnel info in viewport map mode, to prevent render flicker.

If we see a tunnel during the usual tile scan, keep it in the cache
until it is explicitly evicted by a clear tunnel tile action, or
the cache is cleared at game init/load/shutdown.
Select colours at the drawing rather than the cache-building stage.
Bridges and tunnels are now stored/handled separately.
The process for bridges is mostly unchanged.

src/tunnelbridge_cmd.cpp

@@ -40,6 +40,7 @@
 #include "object_base.h"
 #include "water.h"
 #include "company_gui.h"
+#include "viewport_func.h"
 
 #include "table/strings.h"
 #include "table/bridge_land.h"
@@ -866,6 +867,8 @@ static CommandCost DoClearTunnel(TileIndex tile, DoCommandFlag flags)
 			DoClearSquare(tile);
 			DoClearSquare(endtile);
 		}
+		ViewportMapInvalidateTunnelCacheByTile(tile);
+		ViewportMapInvalidateTunnelCacheByTile(endtile);
 	}
 	return CommandCost(EXPENSES_CONSTRUCTION, _price[PR_CLEAR_TUNNEL] * len);
 }