function multiply(out, a, b) {
  var a00 = a[0],
      a01 = a[1],
      a02 = a[2];
  var a10 = a[3],
      a11 = a[4],
      a12 = a[5];
  var a20 = a[6],
      a21 = a[7],
      a22 = a[8];

  var b00 = b[0],
      b01 = b[1],
      b02 = b[2];
  var b10 = b[3],
      b11 = b[4],
      b12 = b[5];
  var b20 = b[6],
      b21 = b[7],
      b22 = b[8];

  out[0] = b00 * a00 + b01 * a10 + b02 * a20;
  out[1] = b00 * a01 + b01 * a11 + b02 * a21;
  out[2] = b00 * a02 + b01 * a12 + b02 * a22;

  out[3] = b10 * a00 + b11 * a10 + b12 * a20;
  out[4] = b10 * a01 + b11 * a11 + b12 * a21;
  out[5] = b10 * a02 + b11 * a12 + b12 * a22;

  out[6] = b20 * a00 + b21 * a10 + b22 * a20;
  out[7] = b20 * a01 + b21 * a11 + b22 * a21;
  out[8] = b20 * a02 + b21 * a12 + b22 * a22;
  return out;
}
Multiplies two mat3's

@param {mat3} out the receiving matrix
@param {mat3} a the first operand
@param {mat3} b the second operand
@returns {mat3} out