This introduces the concept of an #AttributeFilter. It's used to tell a geometry
algorithm which attributes it should process/propagate and which can be ignored.
We already had something similar before named
`AnonymousAttributePropagationInfo`. However, as the name implies, this was
specific to anonymous attributes. This had some downsides:
* A lot of code had to be aware of the concept of anonymous attributes even if
it did nothing special with anonymous attributes.
* For non-anonymous attributes we often had a separate `Set<std::string> skip`
parameter. It's not nice to have to pass two kinds of filters around and to
have to construct a `Set<std::string>` in many cases.
`AttributeFilter` solves both of these downsides.
Technically, `AttributeFilter` could also just be a `FunctionRef<bool(StringRef
attribute_name)>`, but that also has some issues:
* The `bool` return value is often ambiguous, i.e. it's not clear if it means
that the attribute should be processed or not. Using an enum works better.
* Passing function refs around and combining them works, but can very easily
lead to dangling references.
* The default value of a `FunctionRef` is "empty", i.e. it can't be called. It's
generally more nice to not have a special case for the default value. Now the
default `AttributeFilter` propagates all attributes without any extra handling
on the call-site.
Pull Request: https://projects.blender.org/blender/blender/pulls/127155
871b25b219