vtkWindowToImageFilter failed to capture stereo images correctly when
magnification > 1 was employed (or when render was forced). This was
happening because the vtkWindowToImageFilter missed a vtkCamera ivar
(Stereo) in the manual copy. Fixed by using the vtkCamera::ShallowCopy
method instead.

Change-Id: Iacb80794cc34aec1ad53ef0e084fd5f4e4cb2063