touchpad: if a palm touch moves out of the edge zone within a timeout, unpalm

On small touchpads a touch that is intended to traverse much of the screen
width may start at the very edge, i.e. in the palm zone.
In that case, and if the touch moves out of the palm zone quickly enough, drop
the palm label and make it a normal touchpoint.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
filter: move get_direction into shared header

Makes it possible to use from the touchpad code.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
touchpad: require a <45 degree movement for a palm to become a touch

Any legitimate finger movement that starts in the palm area is expected to
move out of the palm area at an angle roughly orthogonal to the edge of the
touchpad. Check for the direction of the movement vector, and if it is within
the accepted cardinal/ordinal directions then proceed.

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