gdk_wayland_*_grab()/ungrab() would emit crossing events which translate
as focus_in/focus_out events for keyboard.
However, the ungrab() functions compare the native toplevel as this is
what gets the Wayland pointer enter/leave events with the grab window,
so if the grab is issued on a child gdk window, those won't match and we
would emit more focus_out events than focus_in events.
This means that a widget such as spice-gtk which issues a keyboard grab
whenever the pointer enters the window and releases the grab when it
leaves the window would get uneven numbers of focus_in/focus_out events.
Also, gdk_wayland_seat_ungrab() would not emit crossing events for
keyboard devices, whereas gdk_wayland_device_ungrab() does, which adds
even more potential discrepancies between focus_in/focus_out events.
To solve this problem, introduce two new helper functions which check
the relevant native windows to emit crossing events when needed that get
called evenly from both gdk_wayland_seat_grab()/ungrab() and gdk_Wayland
_device_grab()/ungrab() APIs.
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=780422
Fixes: https://gitlab.gnome.org/GNOME/gtk/issues/792
cc4dd0d343