Additive AOV's support

[etheory]

I know that this is on your development plan, but I am just publicly re-iterating that mutually-denoised additive AOV's are an essential feature that we require for production rendering at Animal Logic/Netflix Animation Studios.

Our current technique for this is pretty horrid, and whilst it works, it's not what we want to be doing long-term, as whilst it preseves the beauty denoising, the per-channel denoising doesn't look very good.

It's also worth stating that the NVidia OptiX denoiser has this feature (even though the quality of the NVidia OptiX denoiser is nowhere near as good as OIDN for production offline rendering).

Thanks.