Blender Git Loki

Cycles: support OpenImageDenoise in final renders

Performance is not great currently due to the API not seeming to support
efficient denoising of multiple tiles at the same time. So in many cases
only one or a few threads will actually be denoising at the same time.

In renders with many samples this is not a big problem, but for faster
renders it's a signficant overhead.

We should try to optimize this still, possibly by batching denoising of
a bigger neighborhood of multiple tiles at once.