touchpad: if two fingers are within the lower thumb area, they're not thumbs

touchpad: if two fingers are within the lower thumb area, they're not thumbs

The shape of the average hand implies that two fingers down within the lower
thumb area (the bottom few mm of the touchpad) cannot be thumbs without
significant contortion. So let's not mark them as thumb.

Fixes #126

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