BUG: Ensure slice intersection position are updated using VTK6. Fixes #3803

This commit fixes a regression introduced when transitioning to VTK6.

Note that ideally a pipeline should be created, that would avoid to
manually call Update or Modified.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23568 3bd1e089-480b-0410-8dfb-8563597acbee