gestures: average motion by active touches, not moved touches

When two fingers move slowly, an event frame may only have one finger motion,
followed by a frame with the other finger's motion. If we only divide by the
number of dirty touches, the speed of the gesture increases whenever that
happens.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>