BUG: Update VTK - Re-enable GPU VR on Mac fixing a Slicer/VTK rebase mistake

This commit

(1) reverts Slicer specific commit disabling the failing color transfer
function improvements and include the fix done by Cory.

(2) re-enable GPU volume rendering on MacOSX fixing a rebase mistake
introduced in r24461 (ENH: Update VTK to master (soon to be v6.3.0) and also update CTK)

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$ git log --no-merges d645b35..716b0f5
commit 716b0f56fc4bd4b9fcca05f386ec375bcd674122
Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Tue Aug 4 17:49:39 2015 -0400

Fix error preventing GPU volume rendering from working on MacOSX

This commit fixes error introduced when rebasing commit Slicer/VTK@add058e
(Enable GPU Raycast volume mapper for recent NVIDIA drivers on Mac OS X)

commit 74733a0e7d5dd55f5eb52ca411c06ecbb3431ed2
Author: Cory Quammen <cory.quammen@kitware.com>
Date: Thu Jul 23 22:45:26 2015 -0400

Fix crash where vtkColorTransferFunction has 0 nodes

Thanks to Jean-Christophe Fillion-Robin for pointing this out.

See

https://github.com/Slicer/Slicer/commit/f59a689a0f6d2912538ed72dad16efc70601f0ed#commitcomment-9407867

for reference.

A basic test for vtkColorTransferFunction is added that crashes
prior to this patch and succeeds with this patch.

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

commit 7b51a097aead0c872f199a2039d010f28c96e2f5
Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Tue Aug 4 15:08:41 2015 -0400

Re-enable "6657: Add out-of-range colors to color maps"

This reverts commit Slicer/VTK@d645b35.

Thanks: Cory Quammen <cory.quammen@kitware.com>

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git-svn-id: http://svn.slicer.org/Slicer4/trunk@24484 3bd1e089-480b-0410-8dfb-8563597acbee