That's the most likely area it will be resting in, if it's sitting anywhere
above that it's likely part of an interaction.

A thumb in the lowest 15mm needs to trigger the pressure threshold before it's
labelled a thumb. A thumb in the lowest 8mm is considered a thumb if it
remains there for 300ms. Regardless of the pressure, since we can't reliably
get pressure here. If a thumb moves out of the area, or starts outside of that
area it is never a thumb.

If edge scrolling is enabled, the 8mm threshold is ineffective since we'll
have normal interaction in that zone for horizontal scrolling.

The thumb tests now require all touchpads to be switched to clickfinger, if we
test for thumb detection on the bottom of the pad we won't get expected
motion events due to the software button area.

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