To trigger this, we'd need 1, 2, 3 fingers down, release fingers 1 and 2 but
keep 3 down. Then put finger 1 down again. Touches 1 and 3 are alive now,
touch 2 is in state NONE.

During the thumb detection we took the first touch not in BEGIN and assigned
it to "first" - this would now be the second touch in state NONE.

Real effect is relatively minimal since we only use the coordinates here.

Fixes #89



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