wayland: Force the grab cursor while a grab is active

wayland: Force the grab cursor while a grab is active

When a cursor is specified in gdk_seat_grab(), the cursor is reverted as
soon as the pointer enters or leaves another window.

To avoid this issue, store the grab cursor separately, so we force-apply
it in ::set_window_cursor(). Also, unset early the seat info from the
window on gdk_seat_ungrab(), so the next time switch_to_pointer_grab()
happens we end up picking the cursor set for the window underneath the
pointer window.

Based on a patch by Olivier Fourdan <ofourdan@redhat.com>.

https://bugzilla.gnome.org/show_bug.cgi?id=760213