Windows QPA: Fix High DPI scaling not becoming active when changing scaling

When changing scaling from 100%, at which High DPI scaling is inactive,
to something larger,  High DPI scaling does not become active.

Update the factors when screens are refreshed.

Pick-to: 5.15
Task-number: QTBUG-77302
Task-number: QTBUG-77136
Task-number: QTBUG-82267
Change-Id: Iebbc29ced2f23af9db4d67839458848245e2db1d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/windows/qwindowsscreen.cpp

@@ -567,6 +567,7 @@ bool QWindowsScreenManager::handleScreenChanges()
     }     // not lock screen
     if (primaryScreenChanged)
         QWindowsTheme::instance()->refreshFonts();
+    QHighDpiScaling::updateHighDpiScaling();
     return true;
 }