[vtkusers] lib png prob with pygtk2 and vtk 4.3

[David Thompson]

> I am developing an application which uses vtk with pygtk-2.0.0.  When
> I import vtk (whether or not I use it), my stock gtk icons no longer
> work under redhat 9, and I get errors like the following
> 
>   (toolbar_test.py:23182): Gtk-WARNING **: Error loading icon: Failed to load image '/usr/share/icons/Bluecurve/24x24/stock/gtk-new.png': Fatal error in PNG image file: Incompatible libpng version in
> application and library

You should build VTK with VTK_USE_SYSTEM_PNG turned ON. In fact, I turn
all these ON:
VTK_USE_SYSTEM_EXPAT
VTK_USE_SYSTEM_JPEG
VTK_USE_SYSTEM_PNG
VTK_USE_SYSTEM_TIFF
VTK_USE_SYSTEM_ZLIB