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Fix T80771: Avoid changing the visibility of loose geometry when entering Sculpt Mode

When entering scultp mode the visibility from the Face Sets is copied to
the base mesh. This steps was considering that if a vertex belongs to a
face with a visibible Face Set ID, it should be visible. As loose
geometry may not have any faces, those vertex were set to hidden.

Now this function check if a vertex visibility should be modified by the
face sets (by checking the loops), avoiding modifying the visibility of
loose geometry unintentionally.

Reviewed By: sergey

Maniphest Tasks: T80771

Differential Revision: https://developer.blender.org/D8899