All touchpad recordings seen so far show that a value above 100 is definitely
a thumb or a palm. Values below are harder to discern, and the same isn't true
for touchpads supporting ABS_PRESSURE instead of ABS_MT_PRESSURE.

The handling of a touch is as outlined in tp_thumb_detect:
* thumbs are ignored for pointer motion
* thumbs cancel gestures
* thumbs are ignored for clickfinger count
* edge scrolling doesn't care either way
* software buttons don't care either way
* tap: only if thumb on begin

The handling of thumbs while tapping is the simplest approach only, more to
come in follow-up patches.

Note that "thumb" is the synonym for "this touch is too big to be a
fingertip". Which means that a light thumb touch will still be counted as a
finger. The side-effect here is that thumbs resting a the bottom edge of the
touchpad will almost certainly not trigger the pressure threshold because
most of the thumb is off the touchpad.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>