screenshot: Use meta_window_has_pointer () for pointer check

screenshot: Use meta_window_has_pointer () for pointer check

https://gitlab.gnome.org/GNOME/mutter/-/commit/909616b20876478bc56932cd18c8e85e982645f6 seems to have wrapped window actors in a container, so the actor.has_pointer check started failing. Instead, switch to meta_window_has_pointer () which doesn't rely on window actor implementation details.

We check for existence of has_pointer first just in case someone attempts to run gnome-shell 44.1 with mutter 44.0 which does not have the function exported publicly yet.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2710>