function cross(out, a, b) {
  var z = a[0] * b[1] - a[1] * b[0];
  out[0] = out[1] = 0;
  out[2] = z;
  return out;
}
Computes the cross product of two vec2's
Note that the cross product must by definition produce a 3D vector

@param {vec3} out the receiving vector
@param {vec2} a the first operand
@param {vec2} b the second operand
@returns {vec3} out