Improve performance of tunnel rendering in viewport map mode

src/saveload/afterload.cpp

@@ -3774,6 +3774,7 @@ bool AfterLoadGame()
 
 	/* This needs to be done after conversion. */
 	RebuildViewportKdtree();
+	ViewportMapBuildTunnelCache();
 
 	/* Road stops is 'only' updating some caches */
 	AfterLoadRoadStops();