clone function
Creates a new vec3 initialized with values from an existing vector
@param {ReadonlyVec3} a vector to clone @returns {vec3} a new 3D vector
Implementation
List<double> clone(a) {
var out = <double>[];
out[0] = a[0];
out[1] = a[1];
out[2] = a[2];
return out;
}