touchpad: don't check clickfinger distance for three fingers

touchpad: don't check clickfinger distance for three fingers

It's reasonable to expect a thumb (or the other hand's index finger) to click
a button while a finger is down for movement. It's less reasonable to expect
this when two fingers are interacting with the touchpad, or when two fingers
click the touchpad (even on a large touchpad that's an awkward position).

Simplify the clickfinger detection mechanism - if we have three touches down,
it's always a three-finger click. Two fingers may be a right click or a index
+ thumb click.

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