Avoid discarding previous pixmap
When resizing, the SurfaceItem will discard the current pixmap until a new one is created. If the newly created pixmap is valid, the previous pixmap will be discarded in SurfaceItem::updatePixmap().
However, the previous pixmap can be discarded only if it's unreferenced, in other words, no effect needs the previous pixmap.
Edited by Vlad Zahorodnii