libinput: disabling touchpad motion without disabling scrolling?

[Josh Triplett]

Is it possible to disable touchpad motion events without disabling
scrolling?

On a laptop with both a touchstick and a touchpad, I'd like to use a
touchpad as a scroll wheel, but not have it generate any motion events.

(Bonus if I could then scroll with one finger rather than a two-finger
gesture, but that's much less critical.)

If this isn't currently possible, I'd be interested in working on a
patch to fix that.  What would be the most sensible way to integrate
that into libinput's architecture?

- Josh Triplett