function subtract(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;
}
Subtracts matrix b from matrix a

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