gestureswipe: Count last event when calculating velocity

The last event, matching lifting the finger/releaseing the mouse button, is important when there's a large delay between it and the previous events, as in when performing a movement, stopping, then releasing fingers as opposed to doing a swipe.

If this event is skipped, doing this will result in kinetic deceleration matching the previous finger movement, while the expected behavior would be no deceleration.

See also 5dc6194b for a similar fix in GtkEventControllerScroll.