Qt: Redraw window immediately when switching between fullscreen/windowed

Fixes content not showing when displaying a popup message.

src/duckstation-qt/qthostinterface.cpp

@@ -333,6 +333,7 @@ void QtHostInterface::updateDisplayState()
 
   getHostDisplay()->updateImGuiDisplaySize();
   connectDisplaySignals();
+  redrawDisplayWindow();
   UpdateSpeedLimiterState();
 }