Add Distance and Atmospheric Fog support (#6412)

<img width="1392" alt="image" src="https://user-images.githubusercontent.com/418473/203873533-44c029af-13b7-4740-8ea3-af96bd5867c9.png">
<img width="1392" alt="image" src="https://user-images.githubusercontent.com/418473/203873549-36be7a23-b341-42a2-8a9f-ceea8ac7a2b8.png">


# Objective

- Add support for the “classic” distance fog effect, as well as a more advanced atmospheric fog effect.

## Solution

This PR:

- Introduces a new `FogSettings` component that controls distance fog per-camera. 
- Adds support for three widely used “traditional” fog falloff modes: `Linear`, `Exponential` and `ExponentialSquared`, as well as a more advanced `Atmospheric` fog;
- Adds support for directional light influence over fog color;
- Extracts fog via `ExtractComponent`, then uses a prepare system that sets up a new dynamic uniform struct (`Fog`), similar to other mesh view types;
- Renders fog in PBR material shader, as a final adjustment to the `output_color`, after PBR is computed (but before tone mapping);
- Adds a new `StandardMaterial` flag to enable fog; (`fog_enabled`)
- Adds convenience methods for easier artistic control when creating non-linear fog types;
- Adds documentation around fog.

---

## Changelog

### Added

- Added support for distance-based fog effects for PBR materials, controllable per-camera via the new `FogSettings` component;
- Added `FogFalloff` enum for selecting between three widely used “traditional” fog falloff modes: `Linear`, `Exponential` and `ExponentialSquared`, as well as a more advanced `Atmospheric` fog;

Cargo.toml

@@ -297,6 +297,26 @@ description = "A scene showcasing the built-in 3D shapes"
 category = "3D Rendering"
 wasm = true
 
+[[example]]
+name = "atmospheric_fog"
+path = "examples/3d/atmospheric_fog.rs"
+
+[package.metadata.example.atmospheric_fog]
+name = "Atmospheric Fog"
+description = "A scene showcasing the atmospheric fog effect"
+category = "3D Rendering"
+wasm = true
+
+[[example]]
+name = "fog"
+path = "examples/3d/fog.rs"
+
+[package.metadata.example.fog]
+name = "Fog"
+description = "A scene showcasing the distance fog effect"
+category = "3D Rendering"
+wasm = true
+
 [[example]]
 name = "blend_modes"
 path = "examples/3d/blend_modes.rs"

assets/models/terrain/Mountains.gltf

@@ -0,0 +1,143 @@
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+}