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/viewport_func.h

@@ -94,4 +94,7 @@ Point GetViewportStationMiddle(const ViewPort *vp, const Station *st);
 
 void ShowTooltipForTile(Window *w, const TileIndex tile);
 
+void ViewportMapClearTunnelCache();
+void ViewportMapInvalidateTunnelCacheByTile(const TileIndex tile);
+
 #endif /* VIEWPORT_FUNC_H */