Pointclouds skewed while on orthographic projection

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I visualize a pointcloud using `SceneWidget()` first using Perspective projection and then switch to Orthographic projection.

Perspective:

Orthographic:

If you look at the angle between the grid axes, you can see that its not a right angle. What might be reason for this?

Note: I use
`scene.camera.look_at(np.array([-320, -242, 600]), np.array([-800, -350, 800]), np.array([0, -1, 0])) #Center, Eye, Up` to change the camera position