BUG: ConstPointer assigned to non-const raw pointer

itkImageToVectorImageFilter.hxx had a bug that the ConstPointer to the
input image was being assigned to a non-const raw pointer.  This
caused a compilation error.  This is fixed in this patch.

Change-Id: Ibeed0a37752caa60961be7071d99677cc4f87de9
Merge topic 'ImageToVectorBug'

5cd595e BUG: ConstPointer assigned to non-const raw pointer