# Objective

#20838 introduced an `EarthlikeAtmosphere` resource, which has a
pre-prepared scattering medium resource for earthlike atmospheres.

This means that every deployed Bevy app (even apps that don't need an
atmosphere, or don't need the default earthlike atmosphere) will
initialize the earthlike scattering medium (which processes the asset
and writes data to the gpu).

This is not desirable. The UX benefits for atmosphere initializers are
not worth the price being paid:

```rust
// EarthlikeAtmosphere resource
fn setup(mut commands: Commands, earth_atmosphere: Res<EarthlikeAtmosphere>) {
    commands.spawn((
        Camera3d::default(),
        earth_atmosphere.get(),
    ));
}

// Manual initialization
fn setup(mut commands: Commands, mut scattering_mediums: ResMut<Assets<ScatteringMedium>>,) {
    commands.spawn((
        Camera3d::default(),
        Atmosphere::earthlike(scattering_mediums.add(ScatteringMedium::default())),
    ));
}
```

The manual approach also puts the Atmosphere component symbol back at
the forefront and helps instruct developers on how this all actually
works.

## Solution

Remove the EarthlikeAtmosphere resource and add a default
ScatteringMedium impl to make init slightly easier.

As this feature will be introduced in 0.18, to avoid breaking people
down the road we should land this PR in 0.18 as well.