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.