wayland: Set a pixmap while DND

Since our source window isn't moving, let's at least show a copy
of it on the pixmap

A bunch of tearing going on kwin_wayland/nvidia though.
Weston/nvidia is fine.
Haven't tried kwin_wayland/Intel yet

src/private/DragController.cpp

@@ -331,6 +331,7 @@ void StateDraggingWayland::onEntry(QEvent *)
     auto mimeData = new WaylandMimeData();
     QDrag drag(this);
     drag.setMimeData(mimeData);
+    drag.setPixmap(q->m_windowBeingDragged->pixmap());
 
     qApp->installEventFilter(q);
     const Qt::DropAction result = drag.exec();