make screen filtering work with Qt rendering, too

src/frontend/qt_sdl/Screen.cpp

@@ -668,6 +668,8 @@ void ScreenPanelNative::paintEvent(QPaintEvent* event)
 {
     QPainter painter(this);
 
+    painter.setRenderHint(QPainter::SmoothPixmapTransform, filter);
+
     // fill background
     painter.fillRect(event->rect(), QColor::fromRgb(0, 0, 0));
 

src/frontend/qt_sdl/Window.cpp

@@ -761,7 +761,6 @@ void MainWindow::createScreenPanel()
     }
     setCentralWidget(panel);
 
-    actScreenFiltering->setEnabled(hasOGL);
     panel->osdSetEnabled(showOSD);
 
     connect(this, SIGNAL(screenLayoutChange()), panel, SLOT(onScreenLayoutChanged()));