Bugfix in BoxCapsuleDcdContact, contact with box face, not nearest point

If the nearest point on the capsule's segment is not close enough to
contact, and the segment ends before getting close enough to any of
the box's edges, then the point on the capsule segment closest to the
box must be one of its endpoints.
Bugfix in distanceSegmentSegment, small vectors may not be parallel.

A new BoxCapsuleDcdContact unit test exposes a bug in
distanceSegmentSegment, in which the small non-parallel segments
erroneously take the if-then path for parallel segments.