scene/windowitem: only set visible to true if the surface is mapped

While this can't happen with Wayland windows, Xwayland may have a window that's otherwise considered visible but doesn't have a surface (yet, or anymore)

This should fix https://crash-reports.kde.org/organizations/kde/issues/66190/events/e9706586871b4f959aa879e302107bfe/ and a locally reproduce crash (the one I had at Akademy)