fix QWidget::showFullScreen on Windows

When calling SetWindowPos in QWindowsWindow::setWindowState_sys we must
inform the QWidget about the new size we have set.

Task-number: QTBUG-26226

Change-Id: I42b01125f50a94cdb97026c74f5445f9ff47d8ba
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1244,6 +1244,7 @@ void QWindowsWindow::setWindowState_sys(Qt::WindowState newState)
             if (newStates & Qt::WindowActive)
                 swpf |= SWP_NOACTIVATE;
             SetWindowPos(m_data.hwnd, HWND_TOP, r.left(), r.top(), r.width(), r.height(), swpf);
+            QWindowSystemInterface::handleSynchronousGeometryChange(window(), r);
         } else {
             // Restore saved state.
             unsigned newStyle = m_savedStyle ? m_savedStyle : style();