Atmosphere occlusion and PBR shading

[mate-h]

Objective

• Occlude the directional lights by the atmosphere
• More Physically accurate light values reaching the objects from directional lights
• Support volumetric shadowing through the atmosphere with an additional layer of FogVolume
• Strategic direction: use the FogVolume and expand on that implementation for shadow sampling Since it already uses pipeline specialization.
• For now keep the atmosphere pipeline as is, free from shadow sampling code since these effects only matter at large scales. Shadow sampling for the atmosphere can be implemented in a later PR for planetary rings, objects in space, other large scale objects.

Solution

• Bind the transmittance LUT to the core 3d mesh PBR shader
• tint the incoming light L to point P by the transmittance through the atmospheric medium
• Apply the same effects to volumetric fog
• Ensured that new new Atmosphere pipeline key works with deferred rendering
• Updated example to include water plane and screen space reflections. Removed the water plane from the GLB terrain asset. (small asset churn!)

Testing

cargo run --example atmosphere

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