Add: allow setting the highest mountain for heightmaps

It will add some slack to the map height limit if that was set
to auto.

src/heightmap.cpp

@@ -368,7 +368,7 @@ static void GrayscaleToMapHeights(uint img_width, uint img_height, byte *map)
 					/* 0 is sea level.
 					 * Other grey scales are scaled evenly to the available height levels > 0.
 					 * (The coastline is independent from the number of height levels) */
-					heightmap_height = 1 + (heightmap_height - 1) * _settings_game.construction.map_height_limit / 255;
+					heightmap_height = 1 + (heightmap_height - 1) * _settings_game.game_creation.heightmap_height / 255;
 				}
 
 				SetTileHeight(tile, heightmap_height);