Icons that are changed while an actor is not mapped may not have a theme
node associated with, and thus we'd end up not updating them at all.
In fact we return early in st_update_icon(), and this was not an issue
until commit 0b1dfbf6f3f, because we'd end up to update the icon anyways
once the style was changed (and so with a valid theme node), but since
such change we might not updating the icon if no theme detail changed.
To prevent this, add a flag to require an icon update when the theme
changed, if no successfully update happened earlier.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4568
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1983>
2bcc6c09ed