A layer used to combine two parent layers using a whitelist. If the biome from the 
first parent is in the whitelist, it's used, otherwise the biome from the second 
parent is used. This is useful for doing things like combining a heat map and a terrain 
map.



Constructor:
    Arguments:
        long, GenLayer, GenLayer, Non_Null_Biome_ID_Array
    Argument Usage:
        base layer seed, first parent layer, second parent layer, whitelist
    Notes:
        Constructs the layer with its seed, parent layers and whitelist.