Joints as entities

[ThierryBerger]

The current API for (impulse) joints is not idiomatic to ECS:

A body contains its joint data, and contains a reference to its other body. This leads to the following surprises:

• how to choose which body has the joint data ?
• we cannot create 2 joints on a same entity, and have to leverage hierarchy to represent multiple impulse joints.
• API to query joint data is difficult to make ergonomic (see rapier 0.20 feature: RevoluteJoint::angle + more detailed joints enum #530), because data is split between:
  • body1: Entity or other component RigidbodyHandle
  • body2: parent within the component ImpulseJoint.

Suggested change

I suggest moving the ImpulseJoint out of the body, to have a separate entity containing all information: joint data + the linked entities.

This way we could model multiple joints per body and have all joint data available in the same fashion.

To be noted this is also the choice of Avian (see https://docs.rs/avian3d/latest/avian3d/dynamics/solver/joints/struct.FixedJoint.html)