Previously I misunderstood the subsurf modifier's handling of custom
normals. The "use custom normals" check in 4.0 checked if there were
custom normals and whether the auto smooth flag was checked. I wrongly
changed that to check the mesh normals domain instead of whether there
was custom normals. In 4.1, auto smooth isn't required to use custom
normals, but that should be the only change here.
In this PR, that change is done for CPU and GPU subdivision, and for
the versioning which adds a modifier. The versioning now only puts the
new modifier before the subsurf modifier if it would have used the
custom normals interpolation in 4.0.
The last change causes two test failures which I also misunderstood
before. The previous results were arguably incorrect, because the
Cycles experimental adaptive subdivision ignored the auto smooth
angle, which was 5 degrees. It should have been 180 degrees.
I will modify those test files to remove auto smooth from the meshes.
Pull Request: https://projects.blender.org/blender/blender/pulls/119485
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