Improve the handling of coordinate inputs and filters.

Improve the handling of coordinate inputs and filters.

Single-touch and multi-touch coordinate inputs are treated more uniformly,
and the hysteresis filters have a more consistent implementation.  If
possible, pointer control will be assigned to touches with coordinate
updates that pass the default hysteresis filter (the function has been
moved to wsmouse.c).  The "strong" variant of hysteresis has been
improved, the new version won't double the threshold when a movement
changes the orientation on an axis.

There is an additional change in wstpad_configure, which ensures that a
zero size disables an edge area even if the coordinate limits are
misconfigured.