Use same active scalars as parent if possible

For the internal pipeline, try to set the child data sources'
active scalars to match that of the parent if possible. This
prevents the active scalars changing due to operations that
produce child data sources.

Signed-off-by: Patrick Avery <patrick.avery@kitware.com>

Author: psavery

tomviz/operators/OperatorPython.cxx

@@ -657,6 +657,11 @@ void OperatorPython::updateChildDataSource(vtkSmartPointer<vtkDataObject> data)
   if (dataSource->hasTiltAngles())
     dataSource->setType(DataSource::TiltSeries);
 
+  // Keep the same active scalars that the parent had if possible
+  QString prevActiveScalars = this->dataSource()->activeScalars();
+  if (dataSource->listScalars().contains(prevActiveScalars))
+    dataSource->setActiveScalars(prevActiveScalars);
+
   emit dataSource->dataChanged();
   emit dataSource->dataPropertiesChanged();
   ActiveObjects::instance().renderAllViews();