PBRT_V2 总结记录 PointLight 补充（Sample_L） 和 SpotLight 补充（Sample_L）

1. 

Spectrum PointLight::Sample_L(const Scene *scene, const LightSample &ls,
        float u1, float u2, float time, Ray *ray, Normal *Ns,
        float *pdf) const {
    *ray = Ray(lightPos, UniformSampleSphere(ls.uPos[0], ls.uPos[1]),
               0.f, INFINITY, time);
    *Ns = (Normal)ray->d;
    *pdf = UniformSpherePdf();
    return Intensity;
}Spectrum PointLight::Sample_L(cons