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/smallmap_gui.h

@@ -44,7 +44,6 @@ void BuildLinkStatsLegend();
 struct TunnelBridgeToMap {
 	TileIndex from_tile;
 	TileIndex to_tile;
-	uint8 colour;
 };
 typedef SmallVector<TunnelBridgeToMap, 64> TunnelBridgeToMapVector;