function subtract(out, a, b) {
  out[0] = a[0] - b[0];
  out[1] = a[1] - b[1];
  return out;
}
Subtracts vector b from vector a

@param {vec2} out the receiving vector
@param {vec2} a the first operand
@param {vec2} b the second operand
@returns {vec2} out