A QWindow created without an explicit geometry needs to pick up
a default geometry in the platform plugin. If the window has a
maximized of fullscreen window state, it will fill the entire
available geometry of the parent window (or the screen if there's
not parent window).
Fixes: QTBUG-69159
Fixes: QTBUG-69156
Fixes: QTBUG-69154
Pick-to: 5.15
Change-Id: If8565d92a97bb4b3fa44757e68969d54d0bc7ebe
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>