Capsule-capsule coplanar contact generation quality

[RossNordby]

RagdollDemo often ends up with one or two cases of capsules jittering against each other due to rapid changes in the contact manifold. When the capsule internal line segments are coplanar, two contacts are generated, but change between the states can cause instability.

Somewhat difficult. Could expand the contact interval by instead by treating one of the capsule planes as a slab with thickness that the other capsule tests itself against. Thickness would depend on capsule radius and penetration depth.

This is a very old issue and not too terrible, so I'll probably punt it to post-2.4.