Blender Git Loki

Render API/Image Editor: Add postprocessing operator that calls the render engine when executed

This commit adds a general operator for postprocessing render results in the Image editor.
To do so, the render API is extended by two functions:
- can_postprocess checks whether the current render result can be postprocessed by the engine.
For the denoiser, this will check whether the required passes have been rendered.
- postprocess is executed when the user runs the operator. For the denoiser, this will do the actual denoising.