Blender Git Loki

Cycles: Implement Postprocessing callback

This commit finally uses all the work from the earlier commits to implement
the postprocess callback, which triggers a denoising pass on the current
RenderResult.

Note that this does not work on GPUs yet - they can be used for rendering,
but the Device setting has to be switched to CPU rendering before using the
postprocess button.

One other remaining problem is that the Image editor view isn't updated automatically,
you have to switch to another pass/layer and back to see the change.

Also, the feature should eventually be implemeted as a Job to get a progress bar and a
responding UI during denoising.