QMenuProxy: Fix relative positioning with embedded/off-screen parent windows

Without this patch, at least on X11, a menu opened from an embedded
QQuickWidget (which controls a window that's rendered off-screen) may
be mis-positioned for the first time it is shown and until its parent
window is resized in either way.

Co-Authored-By: David Edmundson <kde@davidedmundson.co.uk>