Provide unique id for vtk objects based on memory address

This change allows us to keep track of vtk objects when exporting
a scene description.  It relies on the fact that the memory address
of objects does not change frequently, if ever.

Web/Core/Testing/Python/CMakeLists.txt

 vtk_add_test_python(
   TestDataEncoder.py
   )
+
+vtk_add_test_python(
+  NO_DATA NO_VALID NO_OUTPUT
+  TestObjectId.py
+  )

Web/Core/Testing/Python/TestObjectId.py

+from vtkWebCorePython import vtkWebApplication
+import vtk
+from vtk.test import Testing
+
+class TestObjectId(Testing.vtkTest):
+    def testObjId(self):
+        objId1 = vtkWebApplication.GetObjectId(None)
+        self.assertEqual(objId1, '0')
+
+        polyData = vtk.vtkPolyData()
+        objId2 = vtkWebApplication.GetObjectId(polyData)
+        self.assertEqual(objId2.index('0x'), 0)
+
+        points = polyData.GetPoints()
+        objId3 = vtkWebApplication.GetObjectId(points)
+        self.assertEqual(objId3, '0')
+
+if __name__ == "__main__":
+    Testing.main([(TestObjectId, 'test')])

Web/Core/vtkWebApplication.cxx

@@ -442,3 +442,12 @@ vtkObjectIdMap* vtkWebApplication::GetObjectIdMap()
 {
   return this->Internals->ObjectIdMap.GetPointer();
 }
+
+//----------------------------------------------------------------------------
+const char* vtkWebApplication::GetObjectId(vtkObject* obj)
+{
+  std::ostringstream oss;
+  oss << std::hex << static_cast<void*>(obj);
+  std::string str = oss.str();
+  return str.c_str();
+}

Web/Core/vtkWebApplication.h

@@ -116,6 +116,14 @@ public:
 
   vtkObjectIdMap* GetObjectIdMap();
 
+  /**
+   * Return a hexadecimal formatted string of the VTK object's memory address,
+   * useful for uniquely identifying the object when exporting data.
+   *
+   * e.g. 0x8f05a90
+   */
+  static const char* GetObjectId(vtkObject* obj);
+
 protected:
   vtkWebApplication();
   ~vtkWebApplication();