layout: Exclude hidden actors from struts

Currently we only exclude hidden actors from the input region,
but not from struts. This is an oversight that went unnoticed
until now, probably because we didn't consider the work area
in the overview until recently.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4569

(cherry picked from commit cd1102ff)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1982>