wayland: Fix spectacle not covering fullscreen windows

The most recently activated window can be an overlay window that covers
all screens. If its center is not at an output with the fullscreen
window, then the fullscreen window's stack position won't be lowered. In
order to fix that, this change makes isActiveFullscreen() use
Toplevel::isOnOutput(), which uses geometry info, to check if both windows
are on the same output.