qt: fix `qt-video-autoresize` ill behavior during window state changes

• Currently leaving full screen with qt-video-autoresize causes forgetting the old, overridden size. This is not the behavior of VLC 3. I have observed this on Windows
• Currently leaving full screen with qt-video-autoresize causes the interface to be positioned at (0, 0) on X11. I'm not sure why this happens, as there is only a resize call, but this also fixes that.

Request review @chub.