fix for example: mouse-and-point-coord-py

### 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).

### Describe the issue

I believe there is a small bug in the example "mouse-and-point-coord-py" on line 102.
https://github.com/isl-org/Open3D/blob/master/examples/python/visualization/mouse_and_point_coord.py

Fix: using x,y instead of event.x and event.y when calling unproject as pixel coordinates need to be relative to the scene's widget frame origin

FROM:
```
world = self.widget3d.scene.camera.unproject(
event.x, event.y, depth, self.widget3d.frame.width,
self.widget3d.frame.height)
```

TO:

```
world = self.widget3d.scene.camera.unproject(
x, y, depth, self.widget3d.frame.width,
self.widget3d.frame.height)
```

### Steps to reproduce the bug

1) Create a window with two scenes side by side
2) attach a mouse callback to each scene

left scene will have a correct unproject, while the right scene will not.

this change fixes the problem.