Optimize unit_vector() function?

[hollasch]

Currently defined as

inline vec3 unit_vector(vec3 v) {
    return v / v.length();
}

However, in situations where the vector is already unit length, this can be expensive. We could optimize this with something like

inline vec3 unit_vector(vec3 v) {
    const static auto epsilon = 1e-10;
    const static auto low = 1.0 - epsilon;
    const static auto high = 1.0 + epsilon;

    const auto len_squared = v.length_squared();

    if (low < len_squared || len_squared < high)
        v /= sqrt(len_squared);
}

Try it out to see if there's any significant speedup. Likely there's not, and in this case we should definitely favor simplicity over this fussiness.

[Zor-X-L]

I think it's better to leave it as-is.