scene: Remove addDamage() in SurfaceItem::discardPixmap()
Scene::Window::discardPixmap() used not to add damage. No idea how it got in SurfaceItem::discardPixmap(). Perhaps a mishap when introducing window items.
From logical point of view, adding damage when discarding a pixmap is not necessary too because a new pixmap is going to be created.
Edited by Vlad Zahorodnii