Viewpoint deptdt polygon simplification

For VptDepSimpErr formula see text.
    We derived that, considering the contour, half the number of polygons is required as compared to PolygonSimpErr.
    However for the internal faces, the distortion is 20 times as large as on the contour, thus 10 times PolygonSimpErr.
    We assume smooth transition and use Cos-shaped blending