QWindow::setFlags: return early if the flags didn't change

Prevents calling into QPlatformWindow::setWindowFlags() when
there's no change.

Change-Id: I2dccf3052b5895ee7f05623e6ed468be14bb0c0d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>

src/gui/kernel/qwindow.cpp

@@ -825,6 +825,9 @@ QSurfaceFormat QWindow::format() const
 void QWindow::setFlags(Qt::WindowFlags flags)
 {
     Q_D(QWindow);
+    if (d->windowFlags == flags)
+        return;
+
     if (d->platformWindow)
         d->platformWindow->setWindowFlags(flags);
     d->windowFlags = flags;