function add(out, a, b) {
  out[0] = a[0] + b[0];
  out[1] = a[1] + b[1];
  out[2] = a[2] + b[2];
  out[3] = a[3] + b[3];
  out[4] = a[4] + b[4];
  out[5] = a[5] + b[5];
  return out;
}
Adds two mat2d's

@param {mat2d} out the receiving matrix
@param {mat2d} a the first operand
@param {mat2d} b the second operand
@returns {mat2d} out