enabled gpu volume rendering for Mac

Signed-off-by: Christian Askeland <christian.askeland@sintef.no>
Merge branch 'VTK-5-10-1.patch_branch' of github.com:SINTEFMedisinskTeknologi/VTK into VTK-5-10-1.patch_branch
Merge branch 'VTK-5-10-1.patch_branch' of github.com:SINTEFMedisinskTeknologi/VTK into VTK-5-10-1.patch_branch
clear shared_context in renderwindow on destroy on Mac.