Marble material

[torsteingrindvik]

It would be fun to add procedural materials to the repo.
We could kick it off with some like a marble effect.
A quick search through shadertoy yields cool effects, example: https://www.shadertoy.com/view/Xs3fR4

If we were to go with that approach, we would need to have some things.

#76
#74
#77

Then we'd have to implement the material itself.