ENH: Generate high-resolution magnified screenshots for 3D view. See 0003885

See http://www.na-mic.org/Bug/view.php?id=3885

This commit updates the qMRMLScreenShotDialog to use the class
vtkRenderLargeImage now supporting gradient background since r24668
to do off screen magnified rendering (high resolution) of the 3D render
window when capturing screen shots.

A bug still remains for the 2D render windows in that they get tiled
by this method instead of scaled, so continue using the Qt after capture
scaling for the full view and the slice views.

Reviewed-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Tested-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Nicole Aucoin <nicole@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24670 3bd1e089-480b-0410-8dfb-8563597acbee