Remove duplicate points in vtkDataSetSurfaceFilter for VTK_BEZIER_CURVE

If a curve is composed, for instance, of two linear VTK_BEZIER_CURVE,
the vtkDataSetSurfaceFilter previously created 4 pointsi
(the point between the two elements was duplicated).
This behavior differs from other linear cell types
(e.g., VTK_LINE, VTK_LAGRANGE_CURVE, etc.).
This commit fixes the issue with VTK_BEZIER_CURVE by detecting if a
point has already been processed, instead of systematically creating
new points.