The Lighten mix mode in the Mix RGB node is wrong for factors less than
one. The lighten blend mode is a simple component-wise maximum in EEVEE,
Cycles, and other implementations. While the compositor seems to define
it as a weighted maximum using the factor as the weight. It should be
noted that its Darken counterpart is correctly implemented for the same
node, so Lighten should follow, which this patch do.
GPU compositor shares the EEVEE code, so it is already correct.
Pull Request: https://projects.blender.org/blender/blender/pulls/117630
8b7b165ad9