Bone collection visibility now respects their hierarchy.
A bone collection is only visible when it is marked as visible and all
its ancestors (so parents, greatparents, etc.) are visible. Root bone
collections have no ancestors by definition, and only consider their own
visibility.
The effective ancestors' visibility is stored on each bone collection,
in its `BONE_COLLECTION_ANCESTORS_VISIBLE` flag. This makes it possible
to determine the effective visibility from just the flags of the bone
collection itself.
The `BONE_COLLECTION_ANCESTORS_VISIBLE` flag is now stored, with the
other flags, in `BoneCollection::flags`. This means that it's stored in
DNA, even though it's derived data and should actually be stored in a
runtime struct. However, `BoneCollection` doesn't have any runtime
struct yet, and I don't feel that the introduction of this flag is a
good enough reason to introduce that just yet.
Pull Request: https://projects.blender.org/blender/blender/pulls/116784
6cfbf9ef2f