Blender Git Loki

Cycles: Use the prefiltered shadow feature for denoising

This commit finally adds the prefiltered shadow feature to the main denoising algorithm.
Doing so improves detail preservation a lot: Although the main focus are sharp shadow edges, it actually also helps for Ambient-Occlusion-like and geometric details.

The only issue is that some geometric edges might be a bit noisier after denoising, but that will be fixed in the future by downweighting the shadow feature
when the geometric changes (normals and depth features) are strong.