Tile loop: Add snow line check fast path for tiles well above snow line

src/clear_cmd.cpp

@@ -204,9 +204,13 @@ static void UpdateFences(TileIndex tile)
 static void TileLoopClearAlps(TileIndex tile)
 {
 	int k;
-	if ((int)TileHeight(tile) < GetSnowLine() - 1) {
+	int h = (int)TileHeight(tile);
+	if (h < GetSnowLine() - 1) {
 		/* Fast path to avoid needing to check all 4 corners */
 		k = -1;
+	} else if (h >= GetSnowLine() + 4) {
+		/* Fast path to avoid needing to check all 4 corners */
+		k = 3;
 	} else {
 		k = GetTileZ(tile) - GetSnowLine() + 1;
 	}