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Cycles: combined CPU + GPU rendering support.

CPU rendering will be restricted to a BVH2, which is not ideal for raytracing
performance but can be shared with the GPU. Decoupled volume shading will be
disabled to match GPU volume sampling.

The number of CPU rendering threads is reduced to leave one core dedicated to
each GPU. Viewport rendering will also only use GPU rendering still. So along
with the BVH2 usage, perfect scaling should not be expected.

Go to User Preferences > System to enable the CPU to render alongside the GPU.

Differential Revision: https://developer.blender.org/D2873