touchpad: only identify for pinch in a distinct pinch position

Previously, any lower finger spaced more than the vertical threshold apart
would be labelled as thumb. This causes some taps to be detected as
single-taps, particularly where the user's hand is at an angle that causes the
touches to be effectively vertical.

Restructure that condition so that we only go for a thumb if we're
distinctively apart, and we only *not* go for thumb if we're distinctively
close together.

Fixes #359

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 0ce06d13)