Collider scale

[Jondolf]

Objective

Depends on #154, which needs to be merged first.

Currently, modifying the transform scale of an entity has no effect on its collider. This means that people need to set the collider size to match the size of the scaled mesh, which is often inconvenient. Colliders should get scaled by their transforms like in other game engines such as Godot, Unity and so on.

Solution

Extend Collider to support scaling:

pub struct Collider {
    /// The raw unscaled collider shape.
    shape: SharedShape,
    /// The scaled version of the collider shape.
    ///
    /// If the scale is `Vector::ONE`, this will be `None` and `unscaled_shape`
    /// will be used instead.
    scaled_shape: SharedShape,
    /// The global scale used for the collider shape.
    scale: Vector,
}

Transform scale works in hierarchies the same way as it does for meshes, so scale is propagated to children.

---

Migration Guide

This:

let mesh = meshes.add(Mesh::from(shape::Cube { size: 1.0 }));
commands.spawn((
    PbrBundle {
        mesh,
        transform: Transform::from_scale(Vec3::new(10.0, 1.0, 10.0)),
        ..default()
    },
    Collider::cuboid(10.0, 1.0, 10.0),
    RigidBody::Static,
));

becomes this: (collider size matches unscaled mesh size)

let mesh = meshes.add(Mesh::from(shape::Cube { size: 1.0 }));
commands.spawn((
    PbrBundle {
        mesh,
        transform: Transform::from_scale(Vec3::new(10.0, 1.0, 10.0)),
        ..default()
    },
    Collider::cuboid(1.0, 1.0, 1.0),
    RigidBody::Static,
));