x11: Fix MouseButtonPress events sent to decoration

ButtonPressEvent::state includes keys and buttons prior to the button
press has been generated.

On the other hand, it appears that QMouseEvent::buttons() should include
currently pressed buttons, i.e. the button that has been just pressed
should be in that bitfield.

This is important for QQuickDeliveryAgent, which checks
QMouseEvent::button() and QMouseEvent::buttons() to decide whether to
send the button event only to tap handlers or both tap handlers and
mouse areas.

BUG: 476653