Made icelake placement much fairer so Ice Lake 4 isn't disproportionately spawning compared to the other 3 variants - larger icelakes now spawn more often. Uses a new TerrainAnchorBlendGenerator to generate terrain anchors using the icelake's blend map so anchors are used more sparsely and only in the areas they are needed to make placement easier. The big disc anchors were too mean. Manually reviewed all icelakes across 25 seeds and observed much more diverse placement.