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];
  out[6] = a[6] - b[6];
  out[7] = a[7] - b[7];
  out[8] = a[8] - b[8];
  out[9] = a[9] - b[9];
  out[10] = a[10] - b[10];
  out[11] = a[11] - b[11];
  out[12] = a[12] - b[12];
  out[13] = a[13] - b[13];
  out[14] = a[14] - b[14];
  out[15] = a[15] - b[15];
  return out;
}
Subtracts matrix b from matrix a

@param {mat4} out the receiving matrix
@param {mat4} a the first operand
@param {mat4} b the second operand
@returns {mat4} out