Update the high scaling settings after an XCB screen is added

Without this, the newly-added screen will still have the
scaleFactorProperty unset, which means QScreen::devicePixelRatio will
return 1.0. That differs from what happens if the screen had been
detected when the application started.

This is part of the fix for the bug report, but insufficient.

Task-number: QTBUG-53500
Change-Id: Id3aab65533904562a6cbfffd14502365d86bd36d
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -32,6 +32,7 @@
 ****************************************************************************/
 
 #include <QtGui/private/qguiapplication_p.h>
+#include <QtGui/private/qhighdpiscaling_p.h>
 #include <QtCore/QDebug>
 
 #include "qxcbconnection.h"
@@ -257,6 +258,7 @@ void QXcbConnection::updateScreens(const xcb_randr_notify_event_t *event)
                 } else {
                     screen = createScreen(virtualDesktop, output, outputInfo.data());
                     qCDebug(lcQpaScreen) << "output" << screen->name() << "is connected and enabled";
+                    QHighDpiScaling::updateHighDpiScaling();
                 }
             }
         } else if (screen) {