Ensure all Closures are filled with correct values,
like the principled bsdf node already does.
The main reason is that the new AgX color transform doesn't play well
with negative values (see #113220), but it's probably best to ensure we
use sanitized values in the rendering code as a whole.
Pull Request: https://projects.blender.org/blender/blender/pulls/115059
3f778150a9