Disable camera tracking temporarily, fix bugs

The camera tracking is causing all the physics madness and jitter.
Having an absolute fixed camera *improves* the playability at this point
!
Fix delay before moving when kart stopped

Due to the kart being deactivated.
Enable very simple tracking camera

It's always at a fixed offset behind the kart, so it's not smooth at
all. It's the most stiffest unnatural camera in the history of cameras.
But at least, it doesn't break physics.
Fix and enable improved tracking camera
Mitigate physics madness : limit angular velocity

Currently Bullet seems to think every vertex of the road is something
the kart can crash into and a good reason to make it jump in random
directions.
This patch heavily limits the angular velocity along the X and Z axes to
temporarily workaround this problem.
Default character/kart is now Rarity/TwiCutlass

TwiCutlass is more playable with the crazy physics we have at the
moment.
It's less prone to jumping around at random, and more likely to land on
its wheels after inevitably jumping around.
Add runtime Kart::body nullptr check

It can happen if the .thing script is wrong, and would then cause
undefined behaviours (not always crashes) when using it.
Change a camera update assert into runtime check

This may be a tiny bit jittery when the check fails, but shouldn't be
anything major.
Fix physics madness (due to wrong wheel infos)

Turns out modifying a copy is not very effective.
Remove physics madness mitigation

It's no longer an improvement.
Simplify camera tracking, reduce jitter a little