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Cycles: Viewport Rendering Memory Improvement

Small memory reduction change by only storing the pixels of the combined
pass when it is being shown in the viewport. Previously the combined pass
was always calculated and present in the output buffer. The combined pass
will still be calculated.

It is a limitation in Blender that Cycles always had a combined pass.
This patch will remove the limitation from the code base of Cycles.
Blender still has the limitation, but will always request the combined
renderpass when doing final rendering.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5784