Implement collision margin

[Jondolf]

Objective

Collisions for thin objects such as triangle meshes can sometimes have stability and performance issues, especially when dynamically colliding against other thin objects.

• Infinitely thin objects are more prone to numerical issues and tunneling. Two trimeshes colliding against each other can easily get stuck in an intersecting state or have other collision issues.
• Collision algorithms for intersecting objects are typically more expensive than for the non-intersecting state. Trimesh collisions can be very expensive if the trimesh has dense geometry.

To allow users to mitigate these issues when necessary, engines such as Rapier and Bullet provide a collision margin or contact skin that forces the solver to maintain an artificial separation distance between objects. It can be thought of as a kind of shell that adds thickness to colliders, reducing the issues mentioned above.

Solution

Add a CollisionMargin component. It can be used to add artifical thickness to any collider for collisions.

commands.spawn((
    RigidBody::Dynamic,
    Collider::trimesh_from_mesh(&mesh),
    CollisionMargin(0.05),
));

Without a collision margin:

2024-07-02.22-07-55.mp4

With a collision margin:

2024-07-02.22-09-28.mp4

[janhohenheim]

Suggested change

	collision_margin: Scalar,
	collision_margin: CollisionMargin,

It's weird to refer to CollisionMargin in the docs and then not actually use that type.

[Jondolf]

I don't like having to wrap everything in newtypes unnecessarily so I changed this (and the other similar cases) to impl Into<CollisionMargin> and derived From<Scalar> for now. Not sure if it's worth it though, since the newtypes don't really give an API benefit (other than explicitness and docs) and it just means that we need to call .into() and use derefs in more places internally

[janhohenheim]

Suggested change

	collision_margin: Scalar,
	collision_margin: CollisionMargin,

if we already have the newtypes, we should also be consistently using them imo

[janhohenheim]

This code is duplicated. Consider extracting it into a method.

[janhohenheim]

Math checks out, just some code quality issues, nothing big :)