Blender Git Loki

Compositor: Add OIDN prefiltering option to Denoise node

It's equivalent to the OpenImageDenoise prefiltering option in Cycles.
See D12043.

Prefilter modes:
- None: No prefiltering, use when guiding passes are noise-free.
- Fast: Denoise image and guiding passes together. Improves quality when
guiding passes are noisy using least amount of extra processing time.
- Accurate: Prefilter noisy guiding passes before denoising image.
Improves quality when guiding passes are noisy using extra
processing time.

Reviewed By: #compositing, jbakker, sergey

Differential Revision: https://developer.blender.org/D12342

Compositor: Fix Alpha Over node ignoring emissive colors

It was an issue on Full Frame mode only.

Compositor: Fix crash exporting buffers on debug

ImBuf allocates 4 channels, use copying to support buffers
with 1 and 3 channels.