This patch unifies the Keying node between the CPU and GPU. The
difference was due to imprecision when computing pixel saturation in the
CPU code. This typically happens when the pixel is gray-scale, and
should have a zero saturation, but it came out negative due to
imprecision. To fix this, reorder the mix expression so that the minimum
and maximum values are differenced first.
Ideally, the code shouldn't have such abrupt conditionals and should be
more robust to tiny imprecisions, but that would break the behavior and
should be evaluated separately.
Pull Request: https://projects.blender.org/blender/blender/pulls/118625
a7d4174f8b