cursor: don't leak bound scroll events from touchpad to clients

cursor: don't leak bound scroll events from touchpad to clients

Prior to this commit, when we receive fine-grained scroll events from
touchpads that are bound to any mousebind, we leaked the scroll events to
the client unless the accumulated scroll delta exceeds the fixed threshold.
This was annoying for example when a user wants to ZoomIn/Out with
W-Up/Down mousebinds with a touchpad.

So this commit fixes it by not leaking the scroll events nor executing
actions when the accumulated delta doesn't exceed the threshold.