In an x11 session, we don't receive motion events from X when the
pointer is above a window. Since commit 734a1859 we only do picking on
motion events though, which means when clicking the mouse to focus a
window, we don't repick and might still think the pointer is hovering
above another window or actor, ending up not focussing the window.
Fix this by always repicking on BUTTON_PRESS events. While this is not
necessary in the wayland session, button presses happen rarely compared
to motion events, so it's not a performance regression to do it in
Wayland sessions, too.
Fixes https://gitlab.gnome.org/GNOME/mutter/-/issues/1660
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1752>
4d54c3c556