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 {ReadonlyVec2} a the first operand
@param {ReadonlyVec2} b the second operand
@returns {vec3} out