Certain devices, such as the HTC Flyer, deliver motion events with identical
coordinates before a touch-release event. This causes kinetic panning to fail,
as the calculated velocity is only based on the last two events.
Introduce a velocity change factor, so that the velocity can only change by a
certain amount per event. This has the effect of smoothing velocity changes
and fixes the bug on the HTC Flyer, at least.
