+ Add pickability to `vtkCompositeDataDisplayAttributes` per-block state.
+ Have `vtkGlyph3DMapper` accept a `vtkCompositeDataDisplayAttributes`
  instance named BlockAttributes; use it to specify visibility and
  pickability when the **input** (not source) data is composite.
+ Have `vtkCompositePolyDataMapper2` heed per-block pickability.
+ Test both mappers using hardware selection.