Returns a value of perlin noise given a point.
Used in D1633
It's not a point, it's vector2d. (Widely used inaccuracy, I know, but please change the description to tell that)
See IRC for improvements: http://irclogs.wildfiregames.com/2019-11/2019-11-30-QuakeNet-%230ad-dev.log starting with "12:30 < FeXoR> ...so on to D1630 I guess ^^"