Support custom integration and using LinearVelocity for KCCs

[Jondolf]

Objective

Currently, for KCC movement, users must keep LinearVelocity at zero and manage their own externally stored velocity. Otherwise, you will end up applying the velocity twice, once by the move-and-slide algorithm, and once by the built-in position integration.

This is inconvenient, and also causes potentially incorrect collision response for dynamic bodies that bump into the kinematic body, as the contact won't consider relative velocity properly. Not good!

It would be good if users could use LinearVelocity even for KCCs, and simply change the way it is applied.

Solution

Add CustomVelocityIntegration and CustomPositionIntegration components for disabling Avian's built-in integration logic. This allows users to implement their own movement driven by LinearVelocity and move-and-slide.

Testing

Ran move_and_slide_2d and move_and_slide_3d.

---

Showcase

Add the CustomPositionIntegration component to disable Avian's own position updates based on velocity:

commands.spawn((
    Character,
    RigidBody::Kinematic,
    Collider::from(shape),
    // We want to control position updates manually using move and slide.
    CustomPositionIntegration,
));

Movement systems can then simply modify LinearVelocity. In a way, this also makes many systems generic over dynamic and kinematic character controllers.

fn character_movement(
    mut query: Query<&mut LinearVelocity, With<Character>>,
    input: Res<ButtonInput<KeyCode>>,
) {
    for mut lin_vel in &mut query {
        // Determine movement velocity from input
        let mut delta_vel = Vec2::ZERO;
        if input.pressed(KeyCode::KeyW) {
            delta_vel += Vec2::Y
        }
        if input.pressed(KeyCode::KeyS) {
            delta_vel += Vec2::NEG_Y
        }
        if input.pressed(KeyCode::KeyA) {
            delta_vel += Vec2::NEG_X
        }
        if input.pressed(KeyCode::KeyD) {
            delta_vel += Vec2::X
        }
        delta_vel = delta_vel.normalize_or_zero();
        delta_vel *= 100.0;
        if input.pressed(KeyCode::ShiftLeft) {
            delta_vel *= 2.0;
        }

        lin_vel.0 += delta_vel.adjust_precision();
    }
}

You can then just run move_and_slide in some system, using LinearVelocity as the input velocity, and write the output velocity to LinearVelocity.