function multiply(out, a, b) {
  var a0 = a[0],
      a1 = a[1],
      a2 = a[2],
      a3 = a[3];
  var b0 = b[0],
      b1 = b[1],
      b2 = b[2],
      b3 = b[3];
  out[0] = a0 * b0 + a2 * b1;
  out[1] = a1 * b0 + a3 * b1;
  out[2] = a0 * b2 + a2 * b3;
  out[3] = a1 * b2 + a3 * b3;
  return out;
}
Multiplies two mat2's

@param {mat2} out the receiving matrix
@param {mat2} a the first operand
@param {mat2} b the second operand
@returns {mat2} out