Noise threshhold as well as SPP slider

This would stop rendering when the image gets to a certain noise level, which may be very helpful when used with the animation plugin, where frames in light / dark areas would get different render times

Start by tracing the existing SPP slider and the animation plugin behavior to find where rendering completion is checked. Define how a noise threshold should stop rendering, then verify that frames in light and dark areas can finish at different render times without disrupting the existing SPP limit.

Written by the indexing model from the issue text.