vtkSVGExporter -- definition node issues
<defs> node is not written out, and the fix is in !4754 (closed). This causes images, etc, to be missing the the output.
However, with that patch applied, some issues start popping up when applying both clipping and view transforms, as seen in the
vtkIOExportCxx-TestSVGExporterContextPolyData test. This will require some investigation to properly fix.