Mouse events that are synthesized from touch events do not inherit from QMouseEvent and casting to QMouseEvent crashes application

When `Qt::AA_SynthesizeMouseForUnhandledTouchEvents` is enabled, the unaccepted touch events are sent again as mouse events. However, the synthesized mouse event is not actually a mouse event but a private `QMutableSinglePointEvent` event.

We have code that deals with mouse events and casts incoming mouse events to `QMouseEvent`. Unfortunately, when a mouse event is synthesized from a touch event, this cast fails and the application crashes.

We got around the problem by changing our code so that we cast to `QSinglePointEvent`. But this seems like a bug. I don't think an internal data structure should be exposed like this and when the event type comes in as a mouse event, it's expected that we just cast to a mouse event.