Quality Convergence Indicator

This is a feature request.

As the process of rendering an image is essentially one of convergence to an end state where all light sources have been accounted for I would like some sort of indicator to show how much an image is still changing. This could be useful in judging which SPP targets are necessary for a given image quality.

Suggestions:
1. Sum (RGB components of all pixels)/(3 \* #of pixels) in a frame will converge to a fixed value over time
2. Delta of this value between frames will asymptotically oscillate to approach zero.
3. Maximum of (absolute value for any pixel component ) of this delta will asymptotically approach zero

Ideally this can be deduced in current frame processing with low overhead. One figure (9.99) near the SPP, SPS stats would suffice.

All would vary wildly at first but as SPP increases would settle to a value that could be used as a quality cut-off. e.g. if the average RGB delta is 6 between frames then this would be roughly colour constant at the RGB16 level.

Start by locating the current frame-processing path and the SPP/SPS statistics display mentioned in the issue. Compare the proposed convergence measurements and determine which can be computed with low overhead. Done means agreeing on a defined indicator, displaying it near the existing statistics, and establishing a quality cutoff based on its behavior.

Written by the indexing model from the issue text.