Blender Git Loki

Cycles: Commiting some WIP code for a Quad BVH (CPU only)

* This is highly experimental, but commiting it makes is a bit easier to work on.
* Traversal code in geom_qbvh_traversal.h and geom_triangle.h based on code by Brecht, from the early Cycles days. I updated it, added some AVX2 code and integrated it into the current code base.
* Simple scenes such as cornell box etc. from our test files render ~7% faster, but bigger scenes are quite slower.
* The idea is to use traversal and improved builder code from Embree later.

Differential Revision: https://developer.blender.org/D579