Blender Git Loki

Cycles: Use threads to sort reference arrays when searching for split

This commits implements threaded sorting of references when looking for
object spatial split. It mainly useful when doing initial binning, which
happens from main thread.

Gives nice speedup of BVH build for Bunny.blend: 36sec vs. 55sec for
the Rabbit mesh BVH build.

On more complex scenes the speedup is probably minimal, but still nice
to have more instant rendering for simplier scenes.

Some further tests with production scenes would be interesting.

Reviewers: juicyfruit, dingto, lukasstockner97, brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D1928