'Camera.unproject' does not work properly under KDE-Plasma/Wayland with XDG_SESSION_TYPE=x11

### 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](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [x] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### Describe the issue

The `open3d.visualization.rendering.Camera.unproject` method as demonstrated in the example file _"open3d/examples/visualization/mouse_and_point_coord.py"_ does not work properly when running with XDG_SESSION_TYPE=x11 under KDE-Plasma/Wayland.

When you _Ctrl-click_ on a point of the point cloud the first time, some coordinates are displayed which are incorrect. Every other _Ctrl-click_ on any other point of the point cloud returns exactly the same coordinates.
After zooming or rotating the scene a _Ctrl-click_ does again display different coordinate values. However, again selecting any other point with _Ctrl-click_ displays the same coordinates until you change the view of the scene again.

### Steps to reproduce the bug

```python
export XDG_SESSION_TYPE=x11

python site-packages/open3d/examples/visualization/mouse_and_point_coord.py
```

### Error message

No error message.

### Expected behavior

On _Ctrl-click_ the proper coordinates of the selected point should be displayed.

### Open3D, Python and System information

```markdown
- Operating system: ArchLinux with KDE-Plasma/Wayland
- Python version: Python 3.14.3
- Open3D version: 0.19.0
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
- Compiler version (if built from source):
```

### Additional information

Is it planned to natively support Wayland sometime in the near future?