QXcbDrag: forward mouse release event to initiator window

The mouse release event which ends the drag and drop operation must be
forwarded to the window where the drag started, and not to the
QShapedPixmapWindow, which is not supposed to handle input events.

Task-number: QTBUG-52541
Change-Id: I24a7c7b331a06342ac28b420ff9467aa05fe00dd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>