code for `refract()` in repo doesn't match that of Book 1

[define-private-public]

Which one is the correct one to use?

The book lists this as the code for refract():

vec3 refract(const vec3& uv, const vec3& n, double etai_over_etat) {
    auto cos_theta = dot(-uv, n);
    vec3 r_out_perp =  etai_over_etat * (uv + cos_theta*n);
    vec3 r_out_parallel = -sqrt(fabs(1.0 - r_out_perp.length_squared())) * n;
    return r_out_perp + r_out_parallel;
}

This is what's in the repo right now (https://github.com/RayTracing/raytracing.github.io/blob/master/src/common/vec3.h#L157):

vec3 refract(const vec3& uv, const vec3& n, double etai_over_etat) {
    auto cos_theta = fmin(dot(-uv, n), 1.0);
    vec3 r_out_perp =  etai_over_etat * (uv + cos_theta*n);
    vec3 r_out_parallel = -sqrt(fabs(1.0 - r_out_perp.length_squared())) * n;
    return r_out_perp + r_out_parallel;
}

It's the fmin() for cos_theta. Which one is the correct one to use?

[trevordblack]

the fmin is to guard against NaNs.

This should probably be synchronized across source and text with the fmin

[trevordblack]

Added a PR in #737