Improve playfield swipe detection.

Specifically, accept a small amount of motion the wrong way.  As long
as the deltas one way add up to less than 1/5 of the deltas the other
way, it's recognized as a swipe.  (And the other criteria, which are
unchanged: the X range needs to be more than twice the Y range, the X
range needs to be more than 250, and the time delta needs to be less
than half a second.)