@paper "Smooth Normal Generation with Preservation of Edges" 根据指定角度计算模型法线
refer: * Nate Robins * [email protected] * http://www.pobox.com/~nate * http://titan.cs.ukzn.ac.za/opengl/opengl-d5/trant.sgi.com/opengl/tutorials/examples/smooth/smooth.html
angle - maximum angle (in degrees) to smooth across