main: Make render window borderless fullscreen toggle on the monitor it resides in

Toggling borderless fullscreen on the separate render window made it fullscreen on the monitor which the main yuzu window resided in.

This change allows the render window to go fullscreen on the monitor it resides in, independent of the main window location.

src/yuzu/main.cpp

@@ -2574,7 +2574,7 @@ void GMainWindow::ShowFullscreen() {
 
         render_window->hide();
         render_window->setWindowFlags(windowFlags() | Qt::FramelessWindowHint);
-        const auto screen_geometry = QApplication::desktop()->screenGeometry(this);
+        const auto screen_geometry = QApplication::desktop()->screenGeometry(render_window);
         render_window->setGeometry(screen_geometry.x(), screen_geometry.y(),
                                    screen_geometry.width(), screen_geometry.height() + 1);
         render_window->raise();