copy function
Copy the values from one vec3 to another
@param {vec3} out the receiving vector @param {ReadonlyVec3} a the source vector @returns {vec3} out
Implementation
List<double> copy(List<double> out, a) {
out[0] = a[0];
out[1] = a[1];
out[2] = a[2];
return out;
}