Fix fog color being inaccurate (#10226)

# Objective

Fog color was passed to shaders without conversion from sRGB to linear
color space. Because shaders expect colors in linear space this resulted
in wrong color being used. This is most noticeable in open scenes with
dark fog color and clear color set to the same color. In such case
background/clear color (which is properly processed) is going to be
darker than very far objects.

Example:

![image](https://github.com/bevyengine/bevy/assets/160391/89b70d97-b2d0-4bc5-80f4-c9e8b8801c4c)

[bevy-fog-color-bug.zip](https://github.com/bevyengine/bevy/files/13063718/bevy-fog-color-bug.zip)

## Solution

Add missing conversion of fog color to linear color space.

---

## Changelog

* Fixed conversion of fog color

## Migration Guide

- Colors in `FogSettings` struct (`color` and `directional_light_color`)
are now sent to the GPU in linear space. If you were using
`Color::rgb()`/`Color::rgba()` and would like to retain the previous
colors, you can quickly fix it by switching to
`Color::rgb_linear()`/`Color::rgba_linear()`.

crates/bevy_pbr/src/render/fog.rs

@@ -65,24 +65,33 @@ pub fn prepare_fog(
             match &fog.falloff {
                 FogFalloff::Linear { start, end } => GpuFog {
                     mode: GPU_FOG_MODE_LINEAR,
-                    base_color: fog.color.into(),
-                    directional_light_color: fog.directional_light_color.into(),
+                    base_color: fog.color.as_linear_rgba_f32().into(),
+                    directional_light_color: fog
+                        .directional_light_color
+                        .as_linear_rgba_f32()
+                        .into(),
                     directional_light_exponent: fog.directional_light_exponent,
                     be: Vec3::new(*start, *end, 0.0),
                     ..Default::default()
                 },
                 FogFalloff::Exponential { density } => GpuFog {
                     mode: GPU_FOG_MODE_EXPONENTIAL,
-                    base_color: fog.color.into(),
-                    directional_light_color: fog.directional_light_color.into(),
+                    base_color: fog.color.as_linear_rgba_f32().into(),
+                    directional_light_color: fog
+                        .directional_light_color
+                        .as_linear_rgba_f32()
+                        .into(),
                     directional_light_exponent: fog.directional_light_exponent,
                     be: Vec3::new(*density, 0.0, 0.0),
                     ..Default::default()
                 },
                 FogFalloff::ExponentialSquared { density } => GpuFog {
                     mode: GPU_FOG_MODE_EXPONENTIAL_SQUARED,
-                    base_color: fog.color.into(),
-                    directional_light_color: fog.directional_light_color.into(),
+                    base_color: fog.color.as_linear_rgba_f32().into(),
+                    directional_light_color: fog
+                        .directional_light_color
+                        .as_linear_rgba_f32()
+                        .into(),
                     directional_light_exponent: fog.directional_light_exponent,
                     be: Vec3::new(*density, 0.0, 0.0),
                     ..Default::default()
@@ -92,8 +101,11 @@ pub fn prepare_fog(
                     inscattering,
                 } => GpuFog {
                     mode: GPU_FOG_MODE_ATMOSPHERIC,
-                    base_color: fog.color.into(),
-                    directional_light_color: fog.directional_light_color.into(),
+                    base_color: fog.color.as_linear_rgba_f32().into(),
+                    directional_light_color: fog
+                        .directional_light_color
+                        .as_linear_rgba_f32()
+                        .into(),
                     directional_light_exponent: fog.directional_light_exponent,
                     be: *extinction,
                     bi: *inscattering,

examples/3d/atmospheric_fog.rs

@@ -31,8 +31,8 @@ fn setup_camera_fog(mut commands: Commands) {
             ..default()
         },
         FogSettings {
-            color: Color::rgba(0.1, 0.2, 0.4, 1.0),
-            directional_light_color: Color::rgba(1.0, 0.95, 0.75, 0.5),
+            color: Color::rgba(0.35, 0.48, 0.66, 1.0),
+            directional_light_color: Color::rgba(1.0, 0.95, 0.85, 0.5),
             directional_light_exponent: 30.0,
             falloff: FogFalloff::from_visibility_colors(
                 15.0, // distance in world units up to which objects retain visibility (>= 5% contrast)

examples/3d/fog.rs

@@ -34,7 +34,7 @@ fn setup_camera_fog(mut commands: Commands) {
     commands.spawn((
         Camera3dBundle::default(),
         FogSettings {
-            color: Color::rgba(0.05, 0.05, 0.05, 1.0),
+            color: Color::rgba(0.25, 0.25, 0.25, 1.0),
             falloff: FogFalloff::Linear {
                 start: 5.0,
                 end: 20.0,