function copy(out, a) {
  out[0] = a[0];
  out[1] = a[1];
  out[2] = a[2];
  out[3] = a[3];
  out[4] = a[4];
  out[5] = a[5];
  return out;
}
Copy the values from one mat2d to another

@param {mat2d} out the receiving matrix
@param {mat2d} a the source matrix
@returns {mat2d} out