swaybg process remains after output has been removed

The swaybg process keeps running even if the associated output is long gone. ```sh swaymsg create_output HEADLESS-1 swaybg -o HEADLESS-1 -c #002633 & # do stuff sleep 5 swaymsg output HEADLESS-1 unplug ``` This might be feature rather than a bug and this is certainly not urgent in any way. For a background assigned from the configuration i can see this making sense (swaybg is there when the display is attached). But for instances spawned ad-hoc, this feels more like a leak. If both modes of operation are required, it would be lovely to have a `--linger` option for use with the config. That way, it would be obvious that the lingering is intentional. I assume that for any manual calls the lingering isn't intentional. Changing the default behaviour is going to take a while.