[QtVncServer] Middle mouse button click is not handled when interacting with the app via Vnc server

Steps to reproduce:

1. Open mouse handling example, e.g. https://doc.qt.io/qt-6/qtquick-mousearea-example.html or https://doc.qt.io/qt-6/qtquick-pointerhandlers-example.html 
2. Build and run the app on the desktop with Qt Vnc server
3. Connect via VNC
4. Click the left, right and middle buttons.

Expected: All the clicks are recognized.

Actual: Middle button click is not recognized, only the left and right button clicks are handled. 

Reproducibility:  100%