For unstructured grids, enable point merging when requested

For vtkCutter, point merging is not an explicit option. It is enabled
by setting a vtkPointLocator that merges points. However, if a
vtkNonMergingPointLocator is set as the point locator, then points
will not be merged. Pass along the inferred merging option to
vtk3DLinearGridPlaneCutter when execution is deferred to it.

(cherry picked from commit cbdb93fc)

Filters/Core/vtkCutter.cxx

@@ -405,6 +405,9 @@ int vtkCutter::RequestData(
       newPlane->Push(-d + this->GetValue(0));
 
       linear3DCutter->SetPlane(newPlane);
+      bool mergePoints =
+        this->GetLocator() && !this->GetLocator()->IsA("vtkNonMergingPointLocator");
+      linear3DCutter->SetMergePoints(mergePoints);
       linear3DCutter->SetOutputPointsPrecision(this->GetOutputPointsPrecision());
       linear3DCutter->SetInputArrayToProcess(0, this->GetInputArrayInformation(0));
       vtkNew<vtkEventForwarderCommand> progressForwarder;