demo_convex causes pops.

[Bram Stolk]

When a fragment gets pushed into the groundplane, it often pops out violently.

It seems to be mainly one fragment susceptible to it, although infrequently it may be another one as well? Convex data seems valid.

Does not happen under full-reorder quickstep, but that probably is because it manages to mask the issue with higher stability.

It also happens with the latest version of libccd from upstream github.