function copy(out, a) {
  out[0] = a[0]
  out[1] = a[1]
  out[2] = a[2]
  out[3] = a[3]
  return out
}
Copy the values from one mat2 to another

@alias mat2.copy
@param {mat2} out the receiving matrix
@param {mat2} a the source matrix
@returns {mat2} out