BUG: Update VTKv6 external project - Fix test MarkupsInCompareViewersSelfTest

$ git shortlog 4d7abb2..169b37b
Nicole Aucoin (1):
      BUG: fix null pointer crash in view to world with no active camera

This commit prevent the test from failing but it doesn't avoid the
following error from being displayed:

//------------
338: ERROR: In /home/jchris/Projects/Slicer-2-SuperBuild-Debug/VTKv6/Rendering/Core/vtkRenderer.cxx, line 1252
338: vtkOpenGLRenderer (0x741a1d0): ViewToWorld: no active camera, cannot compute view to world, returning 0,0,0
338:
338:
338: MH Head in background, scrolled to a fiducial
[...]
338: Switched back to conventional layout
338: Switched to Compare grid
338: ERROR: In /home/jchris/Projects/Slicer-2-SuperBuild-Debug/VTKv6/Common/ExecutionModel/vtkAlgorithm.cxx, line 1421
338: vtkOpenGLImageMapper (0x7548770): Attempt to get connection index 0 for input port 0, which has 0 connections.
//------------

See #3761

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23462 3bd1e089-480b-0410-8dfb-8563597acbee