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.
Demo project's source code: quickwidget-menu.zip
Before:
After: Opens up in a correct location on a first try.