Improve disambiguation between two-finger pinch and scroll

Improve disambiguation between two-finger pinch and scroll

A pinch is defined as two fingers moving in different directions, and a
scroll as two fingers moving in the same direction.

Often enough when the user is trying to pinch, we may initially see both
fingers moving in the same direction and decide that they want to
scroll.

Add a grace period during which we may transition to a pinch in those
situations.

Test fix: touchpad_trackpoint_buttons_2fg_scroll emits movements that
change the distance between fingers, which triggers this new transition
and makes the test fail; correct this.

Signed-off-by: novenary <streetwalkermc@gmail.com>