OpenSCADvent day 2 - Perlin noise hillside

thingiverse

**First, an edit:** I added three more modules to [handyfunctions.scad](https://www.thingiverse.com/thing:4671288)! 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](https://www.thingiverse.com/thing:4671288). 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](https://en.wikipedia.org/wiki/Perlin_noise#/media/File:Perlin.png) 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!