antialiased not working for mlab.screenshot

I'm using Mayavi in Jupiter notebook on macOS, and just found that, when using the screenshot function, the results are the same whether the antialiased option is on or not:

```python
# draw something with Mayavi
mlab.figure(size=(500, 500))
obj = mlab.test_plot3d()

# call screenshot
a = mlab.screenshot(figure = obj, mode = 'rgba', antialiased=True)
b = mlab.screenshot(figure = obj, mode = 'rgba', antialiased=False)

# check if they are the same
np.max(a - b)
```

And this returns

```
0
```
which means they are exactly the same. Why is the antialiased option not working?

I searched the Mayavi documentations, and found this

```
obj.scene.anti_aliasing_frames = 8
```

I have tried to change this value, but nothing happened.

By the way, the rendering is pretty good in "x3d backend":

But using screenshot, the result is really a mess:

How to solve this problem?