This actually renders a wallpaper for each output using the newly added
Buffer and BufferPool for shared-memory surfaces and creates a
wlr-layer-shell surface per output. Right now, each wallpaper
shares the same wallpaper (though scaled to each).
wl_output globals get added to a HashMap that is used by Output when it
gets an output event.
Fix null-safety in WindowManager when no seats/outputs exist and route
Window dimensions through pending_manage.
