Blender Git Loki

Cycles: Add option to change which sample to start viewport denoising at

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

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6906