The Nim Ray Tracer Project --- Part 4: Calculating box normals

[I’ve been listening to Shadows of the Heart from Dan Pound all week, it’s just the perfect definition of ambient that, according to Brian Eno, “must be as ignorable as it is interesting”. No track highlights this time, this is really meant to be listened to from beginning to end, over and over again; it’s one singular 55-minute piece of soothing ambient symphony!]
I needed a way to calculate normals for the box primitive in my ray tracer.