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/misc.cpp

@@ -28,6 +28,7 @@
 #include "core/pool_type.hpp"
 #include "game/game.hpp"
 #include "linkgraph/linkgraphschedule.h"
+#include "viewport_func.h"
 
 #include "safeguards.h"
 
@@ -59,6 +60,8 @@ void InitializeGame(uint size_x, uint size_y, bool reset_date, bool reset_settin
 
 	AllocateMap(size_x, size_y);
 
+	ViewportMapClearTunnelCache();
+
 	_pause_mode = PM_UNPAUSED;
 	_fast_forward = 0;
 	_tick_counter = 0;