use mask for denoise node

often you render an image on a transparent background, so only need a small section of the render to be denoised. The OIDN denoise node will denoise the entire image unnecessarily, so it would be great if the denoise node had a mask input to specify which pixels to denoise. This would decrease denoising time dramatically in a lot of cases.

The render region isn't sufficient, because you can't animate it's position like you can with masks.