BUG: Update ITKv4 - Fix GDCMImageIO and TIFFImageIO

The fix related to TIFFImageIO (InsightSoftwareConsortium/ITK@6791b4a3)
should help when building Slicer with option ITK_USE_64BITS_IDS enabled.

// -----------
$ git shortlog c768e9b..056d97b --no-merges
Arnaud Gelas (1):
BUG: GDCMImageIO was not working properly when m_KeepOriginalUID is false

Bradley Lowekamp (1):
BUG: Fix overflows computing size of read tiff image

Matthew McCormick (1):
BUG: gdcm::StringFilter recognizes backslash delimiter.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23716 3bd1e089-480b-0410-8dfb-8563597acbee