Implement Beckmann microfacet distribution model

[abusch]

I think I've got this one in rustracer too.

[wahn]

In C++ (that's what we need in the Rust counterpart):

class BeckmannDistribution : public MicrofacetDistribution {
  public:
    // BeckmannDistribution Public Methods
    static Float RoughnessToAlpha(Float roughness) {
        roughness = std::max(roughness, (Float)1e-3);
        Float x = std::log(roughness);
        return 1.62142f + 0.819955f * x + 0.1734f * x * x +
               0.0171201f * x * x * x + 0.000640711f * x * x * x * x;
    }
    BeckmannDistribution(Float alphax, Float alphay, bool samplevis = true)
        : MicrofacetDistribution(samplevis), alphax(alphax), alphay(alphay) {}
    Float D(const Vector3f &wh) const;
    Vector3f Sample_wh(const Vector3f &wo, const Point2f &u) const;
    std::string ToString() const;

  private:
    // BeckmannDistribution Private Methods
    Float Lambda(const Vector3f &w) const;

    // BeckmannDistribution Private Data
    const Float alphax, alphay;
};