libinput: Only forward first and last press and release for a button

libinput: Only forward first and last press and release for a button

Pointer button events will be received from a device where a button has
been pressed, even though an equivalent button has been pressed (same
button code) on a device connected to the same seat. notify_button()
expects to only be called as if there was only one pointer device
associated with the given seat, so to achieve this, ignore every event
where forwarding it would result in multiple 'pressed' or 'released'
notifications.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>