[Wayland] Menu positioning wrong after window is moved in Weston

On Wayland, using the Weston compositor, if you move the window using Weston's Meta+Drag shortcut, the menu will be incorrectly positioned.

It works as expected in the qwindow compositor example. Note that the method of moving is different: in Weston, one needs to Meta+Drag the window (cf QTBUG-27466), whereas in the example one drags the window by the title bar.

Steps to reproduce:

1. Launch Weston
2. Launch an application with menus (qtcreator or the mainwindow/application example)
3. Click the File menu
4. Move the window (Meta+Drag the window)
5. Click the File menu again

Note how the menu appears there where the it did the first time, instead of at the new position.

This might be a Weston bug.