boxpointer: Fix positioning with non-widget sources

Commit 16d37187 lifted the requirement for source actors to be
widgets, then got it wrong 🙄

The content box is expected to be in actor coordinates, so the
allocation must be translated to (0, 0).

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4593

(cherry picked from commit 0fd205ce)

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