Objective

Resolves #259.
Closes #274.

A common requirement when using joints is to disable collisions between the attached bodies, for example a wheel and a car. Collision layers are typically not suitable for this, as you may still want a car to collide with the wheels of other cars. Implementing the desired behavior properly requires custom hooks, which is both inefficient and overly complicated for such a common scenario.

We should have a first-party JointCollisionDisabled component. To implement it efficiently, we also need to maintain an internal JointGraph to track which joints are connected to which bodies. A joint graph is also needed for #578.

Solution

• Implement a JointGraph that is managed for each joint with a JointGraphPlugin<T: Joint>. To make it work properly, a single entity can now only hold up to one type of joint component. If a joint is added on an entity that already has a joint, the old joint is automatically removed, and a warning is emitted if debug_assertions are enabled.
• Add a JointCollisionDisabled component for disabling collisions between bodies attached by joints.

Migration Guide

Entities can now only hold up to one type of joint component. If a joint is added on an entity that already has a joint, the old joint is automatically removed, and a warning is emitted if debug_assertions are enabled. A rigid body can still be attached to other bodies by multiple joints, but each joint must be on its own entity.

ContactEdge now also stores the rigid body entities.