Blender Git Loki

Cycles X: tweak adaptive sampling threshold

* Fix incorrect handling of samples in adaptive sampling threshold. The previous
formula relied on number of samples cancelling out, but this was not the case.
* Base automatic min samples on the adaptive threshold instead of number of samples,
since we now consider that the main setting to tweak.

Note this changes noise levels in existing renders and requires thresholds to be
tweaked again.

I could not find a significant upside or downsides in scenes when comparing equal
render times. Mainly what this seems to do is make different scenes behave more
consistently with the same noise threshold.

Differential Revision: https://developer.blender.org/D12395