Viewport denoising currently starts right away, which can produce very weird results with a small start resolution. It would be better to only start denoising when it actually makes sense. But based on feedback in D6554 some prefer to have it start right away, I also got feedback that would prefer the opposite though.

To satisfy both views, this patch adds a new user-configurable option to change at which sample viewport denoising should kick in. Setting it to zero retains current behavior (start immediately), while other values will defer denoising until the particular sample has been reached. Default is now at 1, to avoid the weirdness that is AI denoising at small resolutions.