Non-convex colliders should use contact manifolds to improve stability

[Jondolf]

After #90, convex-convex collisions use contact manifolds, i.e. many contact points instead of one. This mitigates pretty much all drifting and explosive behaviour.

However, using the same approach for collisions against non-convex colliders (like trimeshes) caused bodies with convex colliders (like cubes) to sink into the non-convex colliders, sometimes even going straight through. For this reason, they still use the old single contact point method for now.

Fixing this would be really important though, as it would resolve essentially all remaining contact instability that I'm aware of, so let me know if you have any ideas about how to implement it correctly. I'll also look into it more and try to find a fix.