The software button area is currently a partially-dead area. If the finger
moves into or out of the area pointer motion works. Finger motion within the
area however does not generate motion.

The main motivation for this was to avoid accidental pointer motion when a
button is pressed. This is required for stationary fingers but once you move a
significant distance, those bets are off.

So if the finger moves by more than 5mm from where it was put down, release it
and let it move the pointer.

The full impact is largely limited to horizontal movements within the button
area because:
- leaving the finger at the bottom area for 300ms without movement triggers
  the thumb identification, so it won't move anyway.
- moving the finger north is likely to go off the button area before we
  trigger this threshold.

#86



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>