gdk/wayland: Clear modifiers when we lose keyboard focus

When we `Alt+Tab` away from a GTK application, it loses keyboard focus.
If we don't clear the modifiers, events from other devices that we
receive while unfocused will assume `Alt` is still pressed. This results
in e.g. Firefox navigating through the history instead of scrolling the
page when using the mouse wheel on it.

We don't get any information about modifiers while we are missing
keyboard focus, so assuming no modifiers are active is the best we can
do.

The shell sends us a modifier update immediately before we regain
keyboard focus, so the state shouldn't get out of sync.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/2112

gdk/wayland/gdkdevice-wayland.c

@@ -1934,6 +1934,7 @@ keyboard_handle_leave (void               *data,
   g_object_unref (seat->keyboard_focus);
   seat->keyboard_focus = NULL;
   seat->repeat_key = 0;
+  seat->key_modifiers = 0;
 
   GDK_SEAT_NOTE (seat, EVENTS,
             g_message ("focus out, seat %p surface %p",