The use-case of this blend mode is to be able to make parts of an
viewport overlay transparent.
The future user of this blend mode is sequencer preview drawing
where frame will be drawn to an HDR render frame-buffer, and overlays
drawn on-top. In a way it is similar to the image engine, but without
need to have custom shader.
Ref #138094
Pull Request: https://projects.blender.org/blender/blender/pulls/138307
806a633609