COMP: Update Visualization/VTK/Core to use VTK_OVERRIDE

This commit fixes build error like the following:

```
/path/to/CTK/Libs/Visualization/VTK/Core/vtkDiscretizableColorTransferControlPointsItem.h:41:56: error: expected ';' at end of member declaration
/path/to/CTK/Libs/Visualization/VTK/Core/vtkDiscretizableColorTransferControlPointsItem.h:41:58: error: 'override' does not name a type
```

Libs/Visualization/VTK/Core/vtkDiscretizableColorTransferControlPointsItem.h

@@ -38,9 +38,9 @@ public:
     vtkCompositeControlPointsItem)
   static vtkDiscretizableColorTransferControlPointsItem* New();
 
-  bool MouseMoveEvent(const vtkContextMouseEvent &mouse) override;
-  bool MouseButtonReleaseEvent(const vtkContextMouseEvent &mouse) override;
-  bool MouseButtonPressEvent(const vtkContextMouseEvent& mouse) override;
+  bool MouseMoveEvent(const vtkContextMouseEvent &mouse) VTK_OVERRIDE;
+  bool MouseButtonReleaseEvent(const vtkContextMouseEvent &mouse) VTK_OVERRIDE;
+  bool MouseButtonPressEvent(const vtkContextMouseEvent& mouse) VTK_OVERRIDE;
 
   bool IsProcessing();
   void StartProcessing();

Libs/Visualization/VTK/Core/vtkScalarsToColorsContextItem.h

@@ -61,7 +61,7 @@ public:
     const char* xAxisColumn, const char* yAxisColumn);
 
   /// Paint event.
-  bool Paint(vtkContext2D* painter) override;
+  bool Paint(vtkContext2D* painter) VTK_OVERRIDE;
 
   /// Get/Set the color of the current control point.
   void SetCurrentControlPointColor(const double rgb[3]);
@@ -98,7 +98,7 @@ protected:
 
 private:
   vtkScalarsToColorsContextItem();
-  ~vtkScalarsToColorsContextItem() override;
+  ~vtkScalarsToColorsContextItem() VTK_OVERRIDE;
 
   /// Cached geometry of the scene
   vtkVector2i LastSceneSize;