scenes/opengl: Keep lanczos filter after changing outputs

The lanczos filter checks the screen size before rendering in
LanczosFilter::updateOffscreenSurfaces(), so this is not needed.

This simplifies lifetime handling of the lanczos filter, e.g. we
don't need to bother with opengl context anymore, and makes the
scene use less Screens' features.

src/scenes/opengl/scene_opengl.cpp

@@ -585,13 +585,6 @@ void SceneOpenGL::performPaintWindow(EffectWindowImpl* w, int mask, const QRegio
     if (mask & PAINT_WINDOW_LANCZOS) {
         if (!m_lanczosFilter) {
             m_lanczosFilter = new LanczosFilter(this);
-            // reset the lanczos filter when the screen gets resized
-            // it will get created next paint
-            connect(screens(), &Screens::changed, this, [this]() {
-                makeOpenGLContextCurrent();
-                delete m_lanczosFilter;
-                m_lanczosFilter = nullptr;
-            });
         }
         m_lanczosFilter->performPaint(w, mask, region, data);
     } else