Add Stochastic Screen-Space Reflections

## What problem does this solve or what need does it fill?
Reflections technique that robustly handles spatially-varying material properties, reproduces specular elongation of microfacet BRDFs.

1. Sharp and blurry reflections
2. Contact hardening
3. Specular elongation

Example from talk:

## Additional context
Talk by Tomasz Stachowiak at SIGGRAPH 2015
(https://www.ea.com/frostbite/news/stochastic-screen-space-reflections)
(https://www.slideshare.net/slideshow/stochastic-screenspace-reflections/51840375)