vtkLegendScaleActor: add Modified() call after updating points

After updating the points of the vtkPoints instance used for the legend, it's
necessary to call Modified(). Otherwise, the legend doesn't update its size or
position when the window is resized. The LegendScaleActor example exhibits this
behavior:

http://www.vtk.org/Wiki/VTK/Examples/Cxx/Visualization/LegendScaleActor

Rendering/Annotation/vtkLegendScaleActor.cxx

@@ -399,6 +399,7 @@ void vtkLegendScaleActor::BuildRepresentation(vtkViewport *viewport)
       this->LegendPoints->SetPoint(7, x1+2*delX,20,0);
       this->LegendPoints->SetPoint(8, x1+3*delX,20,0);
       this->LegendPoints->SetPoint(9, x1+4*delX,20,0);
+      this->LegendPoints->Modified();
 
       // Specify the position of the legend title
       this->LabelActors[5]->SetPosition(0.5*size[0],22);