[vtkusers] Fix Ubuntu 14.04 VTK 6.1 compiling issue

[Severin Holzer-Graf]

Hi,

I faced the same compiling issue on Ubuntu 14.04 as mathew here:
http://www.vtk.org/pipermail/vtkusers/2014-August/084877.html
Compiling fails, because "GLsizeiptr has not been declared".

The issue is that "vtkgl.h" is included *after* "GL/glx.h" which needs
the typedefs for the datatypes from "vtkgl.h".
I simply swapped the two lines and VTK compiles. Appended a patch that
shows the change.

Interestingly on my ArchLinux system compiling VTK 6.1 works out of
the box and this was not necessary. I didn't further investigate that,
so I'm not sure if swapping the two lines is actually the correct fix.

Best
Severin
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