With the OptiX viewport denoiser active, an OptiX device was added to the device list even when
rendering and denoising on different devices (e.g. CPU or CUDA) in background rendering. This
fixes it, so the OptiX device is only added when actually needed, as in only when OptiX denoising is
actually active in background rendering.
The OptiX denoiser can be a great help when rendering in the viewport, since it is really fast
and needs few samples to produce convincing results. This patch therefore adds support for
using any Cycles denoiser in the viewport also (but only the OptiX one is selectable because
the NLM one is too slow to be usable currently). It also adds support for denoising on a
different device than rendering (so one can e.g. render with the CPU but denoise with OptiX).
Reviewed By: #cycles, brecht
Differential Revision: https://developer.blender.org/D6554
