Fix ball penetrating arena under sustained force

[Z-OMG]

No description provided.

[ZealanL]

Interesting... how does this change affect collision accuracy? Is there a reproducible case for this? How easily can this bug occur? Many questions...

[Z-OMG]

RocketSim V3 has issues with the ball penetration, it is a fact and its tested.
V3's contact solver did

if cp.is_special { …; continue; }

for ball-vs-world contacts, skipping the per point constraint that carries the penetration depth.
So the ball gets zero positional recovery and sinks into the floor/wall under sustained force.
It is tested in the latest version of RocketSim V3.
I tested it by putting a sustained force on the ball towards the ground, then it sank underground.

[Z-OMG]

And what I did to fix it:

1. Accumulate the real penetration alongside the existing sums: total_penetration += cp.distance_1

2. In convert_contact_special, feed the averaged real penetration (total_penetration/num) into the split-impulse positional term of that single averaged constraint.

[VirxEC]

Are we sure let distance = sri.total_dist / num_collisions; shouldn't just be replaced with avg_penetration

[Z-OMG]

No, distance has to stay total_dist / num_collisions. It's used for two unrelated things, and only one of them is the penetration:

1. rel_pos1 = normal_world_on_b * -distance

2. let penetration = distance;

distance = radius (contact geometry), avg_penetration = real depth (positional recovery). They're not interchangeable, so merging them regresses the contact solve.