"Run once" Scenes

[cart]

Objective

Scene and SceneList currently require "repeatability". This puts some burden on developers setting "field patches" in BSN, as the "right hand side" values must either be Copy or have a manual Clone. This results in scenes like:

let mesh = meshes.add(Circle::new(4.0));
let material = materials.add(Color::WHITE);

bsn! {
    Mesh3d({mesh.clone()})
    MeshMaterial3d::<StandardMaterial>({material.clone()})
}

In practice, thanks to inheritance caching, repeatability is no longer required.

Solution

• Make Scene and SceneList consume self.
• Add SceneBox and SceneListBox subtraits to enable Box<dyn Scene> to impl Scene and consume itself.
• Switch to FnOnce for patches.

This enables the following code to work:

let mesh = meshes.add(Circle::new(4.0));
let material = materials.add(Color::WHITE);

bsn! {
    Mesh3d(mesh)
    MeshMaterial3d::<StandardMaterial>(material)
}

[alice-i-cecile]

Approved with minor feedback. Feel free to fix or ignore.

The elimination of the clones in your sample code really clearly highlights the value here.

[MalekiRe]

The code LGTM on a functional level