Fix z-check for flat road foundations

src/road_cmd.cpp

@@ -2478,10 +2478,15 @@ static void TileLoop_Road(TileIndex tile)
 {
 	switch (_settings_game.game_creation.landscape) {
 		case LT_ARCTIC:
-			if (IsOnSnow(tile) != (GetTileZ(tile) > GetSnowLine())) {
+			{
+				/* Flat foundation tiles should look the same as the tiles they visually connect to. */
+				int tile_z = GetFoundationSlope(tile) == SLOPE_FLAT ? GetTileMaxZ(tile) : GetTileZ(tile);
+
+				if (IsOnSnow(tile) != (tile_z > GetSnowLine())) {
 					ToggleSnow(tile);
 					MarkTileDirtyByTile(tile);
 				}
+			}
 			break;
 
 		case LT_TROPIC: