WIP: Cloud scattering prototype

[mate-h]

Objective

• Add clouds to the atmosphere

Solution

• Add cloud layer bindings, fbm noise
• Alternative solution 1 (preferred): use the FogVolume component to create the cloud layer and improve the rendering logic there.
• Alternative solution 2: use a new ScatteringTerm and the generalized atmosphere parameters with a custom texture somehow.
• Make sure this also works with the FogVolume rendering which is localized volumes for the scene itself.
• Limitations: only renders in ray-marched mode, and does not render in the sky-view lut accelerated mode. Not performance optimized, noisy / no TAA

Testing

• Run atmosphere example

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Showcase

Current state:

• lighting works well with cloud shadow map and filtering pass
• reflection probe integration works (via generated envmap path)
• cloud shaping is still a bit WIP, needs proper 3d noise for detail erosion / breakup, and density profiles needs fine-tuning
• still some visible noise at lower sample counts. time to introduce STBN so noise can be removed with TAA
• needs PBR pipeline integration (cloud shadow affects ground)

commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-2.4, 0.04, 0.0).looking_at(Vec3::Y * 0.1, Vec3::Y),
        // get the default `Atmosphere` component
        earth_atmosphere.get(),
        // Can be adjusted to change the scene scale and rendering quality
        AtmosphereSettings::default(),
        AtmosphereSettings {
            rendering_method: AtmosphereMode::Raymarched,
            ..default()
        },
        CloudLayer {
            cloud_layer_start: 6_362_000.0,
            cloud_layer_end: 6_363_000.0,
            cloud_density: 1.0,
            cloud_absorption: 0.00005, // m^-1
            cloud_scattering: 0.0008, // m^-1
            noise_scale: 64_000.0,
            noise_offset: Vec3::ZERO,
            detail_noise_scale: 16_000.0,
            detail_strength: 1.0,
        },
        //...
))