Cycles: Add support for denoising in the viewport

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

intern/cycles/render/buffers.cpp

@@ -146,7 +146,7 @@ void RenderBuffers::reset(BufferParams &params_)
   params = params_;
 
   /* re-allocate buffer */
-  buffer.alloc(params.width * params.height * params.get_passes_size());
+  buffer.alloc(params.width * params.get_passes_size(), params.height);
   buffer.zero_to_device();
 }