Blender Git Loki

Cycles: add support for BVH refit in OptiX

This avoids recomputing the BVH for geometries that do not have changes in topology but whose vertices are modified (like a simple character animation), and gives up to 40% speedup for BVH building.

This is only available for viewport renders at the moment.

Reviewed By: pmoursnv, brecht

Differential Revision: https://developer.blender.org/D9353