function add(out, a, b) {
  out[0] = a[0] + b[0];
  out[1] = a[1] + b[1];
  return out;
}
Adds two vec2's

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