wayland: Fix spectacle not covering fullscreen windows

Vlad Zahorodnii requested to merge work/zzag/active-full-screen-overlay into master

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.

Spectacle will stop creating one giant overlay window with graphics/spectacle!125 (closed), which is needed to prepare it for the layer-shell protocol on wayland.

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