Question: don't we need to do something to back out the penetration or the speeds that the two particles are moving here? In other words, if two particles have collided and one of them was stationary, and we detect the collision after the penetration occurred, then it seems like the contact point is not really at the center of the two particles, but instead is something back further in time on the path of the moving particle, and closer to the original edge of the stationary particle?

I don't want to hang up this PR for this question, so I will likely merge even with this question. But I wanted to ask this because I think there's a chance to make this better. Thoughts?