OpenSCADvent day 2 - Perlin noise hillside

First, an edit: I added three more modules to handyfunctions.scad! xy(), xz(), and yz() fill the positive plane, which is really useful for difference() and intersection() operations.

Here is a pleasant hillside surface generated from Perlin noise. It's a good demonstration of the extremely useful modules from Day 1. In proper parametric form, you can change the angle of the hillside. This was going to be a bit more elaborate, but then I discovered that the surface() command can't take a matrix as input, it has to take an external file. This seems like an easy feature that OpenSCAD could implement! Anyway, I used the sample Perlin noise bitmap from Wikipedia to create a flat surface. Then I provided two slightly different approaches to angle it upward without too much distortion.

Yes, it's a very simple SCADvent today, but it's one that future SCADvents will rely on, so I wanted to get it in early!