COMP: Restore TestOffAxisStereo on Mac

The TestOffAxisStereo test was recently removed from Mac builds
because of multi-month, recurring failures. In a recent topic,
http://review.source.kitware.com/#/t/2193/ Aashish fixed issues with
WindowToImageFilter that prevented some stereo tests from getting
proper test images. This ended up TestOffAxisStereo which were also
fixed.

Change-Id: I6916219969520c688acdad3831ec8ecd96c224f7
Merge topic 'RestoreTestOffAxisStereo' into master

fc3db726 COMP: Restore TestOffAxisStereo on Mac