Blender Git Loki

Cycles: CUDA faster rendering of small tiles, using multiple samples like OpenCL.

The work size is still very conservative, and this doesn't help for progressive
refine. For that we will need to render multiple tiles at the same time. But this
should already help for denoising renders that require too much memory with big
tiles, and just generally soften the performance dropoff with small tiles.

Differential Revision: https://developer.blender.org/D2856