vnc platform: mouse wheel doesn't work

Run any example that ought to have some sort of mouse wheel interaction with -platform vnc. Connect with vncviewer to port 5900. Move the wheel. If you have added a qDebug in QVncClient::pointerEvent(), it's possible to see that the wheel events get turned into mouse move events.

The mouse button from the QRfbPointerEvent is 0 (no button). But we only implement RFB 3.3 so far; the specification says nothing about wheel events. Version 3.7 of the spec says they are represented by press/release of mouse buttons 4 and 5 (as on classic X11). So we are compliant with the spec that we implemented, but it's possible to fix this by implementing a newer spec.

The latest spec is 3.8, AKA RFC 6143: https://tools.ietf.org/html/rfc6143