Ray marching / Distance Fields

Might be similar to https://github.com/CodingRainbow/Rainbow-Topics/issues/14 (ray marching, marching cubes, ray casting, ray tracing... what is the difference?)

Mind was blown by this talk: [NVScene 2015 Session: How to Create Content with Signed Distance Functions (Johann Korndorfer)](https://www.youtube.com/watch?v=s8nFqwOho-s)
but there's an assumption that the viewer knows some basic things about GLSL

ref:
http://iquilezles.org/www/articles/distfunctions/distfunctions.htm
http://www.iquilezles.org/www/articles/terrainmarching/terrainmarching.htm
http://www.viz.tamu.edu/faculty/ergun/research/implicitmodeling/papers/sm99.pdf
http://www.hugi.scene.org/online/hugi37/hugi%2037%20-%20coding%20adok%20on%20ray%20casting,%20ray%20tracing,%20ray%20marching%20and%20the%20like.htm