BUG: Fix memory initialization in vtkMRMLGlyphableVolumeDisplayNode

This commits fixes a problem discovered while running the test
vtkMRMLGlyphableVolumeDisplayNodeTest through valgrind. It ensures
that the ivar "VisualizationMode" is initialized.

[...]

==32528== Conditional jump or move depends on uninitialised value(s)
==32528== at 0x23682EF0: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.18)
==32528== by 0x236834BC: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.18)
==32528== by 0x2368EFAD: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.18)
==32528== by 0x53A4422: vtkMRMLGlyphableVolumeDisplayNode::PrintSelf(std::ostream&, vtkIndent) (vtkMRMLGlyphableVolumeDisplayNode.cxx:153)
==32528== by 0x17BF4BD7: vtkObjectBase::Print(std::ostream&) (vtkObjectBase.cxx:149)
==32528== by 0x508DBD: vtkMRMLGlyphableVolumeDisplayNodeTest1(int, char**) (vtkMRMLGlyphableVolumeDisplayNodeTest1.cxx:22)
==32528== by 0x4729AC: main (MRMLCoreCxxTests.cxx:616)

[...]

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23458 3bd1e089-480b-0410-8dfb-8563597acbee