Blender Git Loki

Cycles: Improvements for the resolution divider calculation

* Do not report render and denoising time if render has been cancelled.

It is not possible to estimate full rendering time from a cancelled
state, and reporting wrong time gives the scheduler the wrong idea
about how fast render is.

* Use timing of the first sample to calculate start resolution divider.

This timing is more accurate, because in viewport is known to include
a single sample, and it resembles performance better because later on
multiple samples might be scheduled, making it seen that render is
faster.

* Add some tolerance to the desired timing, so that if the higher
resolution gives close enough timing we use higher resolution. This
solves resolution divider bouncing between two values on a boundary
values of render time (however, it does not eliminate requirement of
hysteresis to really ensure this).

Some utilities became public, so that they can be covered with the
tests. This is mainly to verify own mental math, but also to allow to
play with analytical formulas without breaking things.