For mipmapped environment maps, create the mipmaps before calling bmap_to_cubemap (like bmap_to_cubemap(bmap_to_mipmap(skymap)). Creating mipmaps afterwards will produce an invalid environment map.
expliciptly states, that one should create mipmaps before cubemaps and that doing so after creating cubemaps produces an invalid cubemap.
Which it does, if i do
bmap_to_mipmap(bmap_to_cubemap(skymap))
i get a black cubemap.
