Gravity and collision
How hard everything is pulled down — and what bumps into what.
Gravity
Its own strength per avatar. Applies to every chain (hair, ears, tail) and to accessory pendulums.
Collision groups
An object belongs to one or more groups; only things in the same group collide.
Collision groups also decide which objects particles can hit.
TODO: Write out an example with two groups (hair against body, particles against the floor).
Reset position
Puts the avatar back when physics or tracking has moved it.