xwayland: Use relative device for buttons/axis

We are using the relative pointer for motion events, but buttons and
axis events still go through the absolute pointer device.

That means additional DeviceChanged events that could be avoided if the
buttons and axis events were coming from the same device as motion
events.

Route those events to the relative pointer if available so that motion,
buttons and axis events come from the same device (most of the time).

Suggested-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Related: xorg/xserver#1130
4 jobs for relative-motion-events in 5 minutes and 55 seconds (queued for 1 second)