STYLE: vtkSegmentationCore: Fix sign-compare warnings

This commit fixes warnings like these ones:

/path/to/Slicer/Libs/vtkSegmentationCore/vtkSegmentationHistory.cxx: In static member function ‘static void vtkSegmentationHistory::OnSegmentationModified(vtkObject, long unsigned int, void, void*)’:
/path/to/Slicer/Libs/vtkSegmentationCore/vtkSegmentationHistory.cxx:368:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (self->LastRestoredState != self->SegmentationStates.size())
^

/path/to/Slicer/Libs/vtkSegmentationCore/vtkSegmentation.cxx: In member function ‘vtkSegment* vtkSegmentation::GetNthSegment(int) const’:
/path/to/Slicer/Libs/vtkSegmentationCore/vtkSegmentation.cxx:614:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (index < 0 || index >= this->SegmentIds.size())
^

vtkOrientedImageDataResample.cxx: In instantiation of ‘void MergeImageGeneric2(vtkImageData, vtkImageData, int, const int, int, double) [with BaseImageScalarType = double; ModifierImageScalarType = long long unsigned int]’:
/path/to/Slicer/Libs/vtkSegmentationCore/vtkOrientedImageDataResample.cxx:209:5: required from ‘void MergeImageGeneric(vtkImageData, vtkImageData, int, const int, int, double) [with BaseImageScalarType = double]’
/path/to/Slicer/Libs/vtkSegmentationCore/vtkOrientedImageDataResample.cxx:1032:5: required from here
/path/to/Slicer/Libs/vtkSegmentationCore/vtkOrientedImageDataResample.cxx:176:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (*modifierImagePtr > maskThreshold)
^

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25594 3bd1e089-480b-0410-8dfb-8563597acbee