touchpad: require minimum scroll distance and lock scroll direction

touchpad: require minimum scroll distance and lock scroll direction

This is a fairly rough approach, but can be handled more fine-grained later.
Require a minimum of 1 unit to start scrolling and lock the scrolling in the
initial direction, so further scroll events are limited to that direction
only.

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