deformation grid does not working if warp image loaded as an overlay

[jilei-hao]

Original Issue: https://github.com/pyushkevich/itksnap/issues/106

[jilei-hao]

Cause:

• Overlay context actors were not added to the renderer when displayed as overlays
• Renderer viewport were not updated for overlays either so context actor transforms were incorrectly updated

[jilei-hao]

addressed: https://github.com/jilei-hao/itksnap/pull/171