Move static data in vtkVRMLImporter to local struct

The bug is as follows:  Create a vtkVRMLImporter and load a file. Delete
your importer.  Create a new importer and try to load a file... and
it will segfault since the destructor deletes the static pointers and
doesn't reset the other static data.  This commit adds a test for this
bug too.

Change-Id: Ie9b9e878035198efad20275f168d36222e63715e
Merge topic 'fix_vrml_importer_segfault' into master

011ce9b2 Move static data in vtkVRMLImporter to local struct