Blender Git Loki

Cycles: add cycles.merge_images operator for combing EXR renders.

This is only available through the API, mainly intended for render farms to
combine rendered multilayer EXR Files with different samples. The images are
currently expected to have the exact same render layers and passes, just with
different samples.

Variance passes are still simply a weighted average, ideally these should be
merged more intelligently.

Differential Revision: https://developer.blender.org/D4554