I intend to add a new feature BottomEdgeZones which reuses the Zone
machinery, but is going to use a different source for the effective X
coordinate which selects the zone.
Break out the computation of `pos'. pos==-1 means the zone feature is
not in use or not applicable to the event.
No functional change in this commit.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>