Force a delay before image capture in volTM2DRotateClip.

Some platforms seem to capture an image of the volume before it has
finished rendering for the last time.  Introduce a delay as an
experiment to see if this changes the captured image.

Change-Id: I71213b651acca2d8c9a8c4f42d147533736da596
Merge topic 'vtkTM2DRotateClip-delay' into master

fd54bd37 Force a delay before image capture in volTM2DRotateClip.