QWizard: Correctly calculate watermark size hint

If we have have AA_EnableHighDpiScaling on and have loaded a @2x image, layout
calculations can't use the real pixels of the QPixmap directly.

Fixes: QTBUG-73401
Change-Id: I1891411a0e359e0148476f73b6cc3a128893a374
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/dialogs/qwizard.cpp

@@ -456,7 +456,7 @@ public:
 
     QSize minimumSizeHint() const override {
         if (pixmap() && !pixmap()->isNull())
-            return pixmap()->size();
+            return pixmap()->size() / pixmap()->devicePixelRatio();
         return QFrame::minimumSizeHint();
     }