// Rotates the vector around another vector by a given angle.
    void rotate(vec3 around, float angle) {
        // http://inside.mines.edu/fs_home/gmurray/ArbitraryAxisRotation/
        float u, v, w, d, c, ci, s;
        vec3 newVector(0, 0, 0);
        u = around.x;
        v = around.y;
        w = around.z;
        d = x * around.x + y * around.y + z * around.z;
        c = dcos(angle); ci = 1 - c;
        s = dsin(angle);
        newVector = vec3(
            u * d * ci + x * c + (-w * y + v * z) * s,
            v * d * ci + y * c + (w * x - u * z) * s,
            w * d * ci + z * c + (-v * x + u * y) * s
        );
        x = newVector.x;
        y = newVector.y;
        z = newVector.z;
    }