- The vtkHardwareSelector::Render method now renders only the
  actors whose mappers are compatible with the current selector pass.
- Updated the internal implementation of vtkHardwareSelector to be able
  to generate a single vtkSelection object from pixel information that
  is mixed with cellgrid and dataset instances.
- Uses the actor ID from the ACTOR_PASS to retrieve the actor and
  determine whether a pixel was drawn by a vtkCellGridMapper or not.
- Adds a new unit test which exercises the hardware selector in a
  renderer with both a vtkPolyData and a vtkCellGrid.