Send emissive color to uniform as linear instead of sRGB (#7897)

This produces more accurate results for the `EmissiveStrengthTest` glTF test case.

(Requires manually setting the emission, for now)

Before: <img width="1392" alt="Screenshot 2023-03-04 at 18 21 25" src="https://user-images.githubusercontent.com/418473/222929455-c7363d52-7133-4d4e-9d6a-562098f6bbe8.png">

After: <img width="1392" alt="Screenshot 2023-03-04 at 18 20 57" src="https://user-images.githubusercontent.com/418473/222929454-3ea20ecb-0773-4aad-978c-3832353b6871.png">

Tagging @JMS55 as a co-author, since this fix is based on their experiments with emission.

# Objective

- Have more accurate results for the `EmissiveStrengthTest` glTF test case.

## Solution

- Make sure we send the emissive color as linear instead of sRGB.

---

## Changelog

- Emission strength is now correctly interpreted by the `StandardMaterial` as linear instead of sRGB.

## Migration Guide

- If you have previously manually specified emissive values with `Color::rgb()` and would like to retain the old visual results, you must now use `Color::rgb_linear()` instead;
- If you have previously manually specified emissive values with `Color::rgb_linear()` and would like to retain the old visual results, you'll need to apply a one-time gamma calculation to your channels manually to get the _actual_ linear RGB value: 
  - For channel values greater than `0.0031308`, use `(1.055 * value.powf(1.0 / 2.4)) - 0.055`;
  - For channel values lower than or equal to `0.0031308`, use `value * 12.92`;
- Otherwise, the results should now be more consistent with other tools/engines.

crates/bevy_pbr/src/pbr_material.rs

@@ -401,7 +401,7 @@ impl AsBindGroupShaderType<StandardMaterialUniform> for StandardMaterial {
 
         StandardMaterialUniform {
             base_color: self.base_color.as_linear_rgba_f32().into(),
-            emissive: self.emissive.into(),
+            emissive: self.emissive.as_linear_rgba_f32().into(),
             roughness: self.perceptual_roughness,
             metallic: self.metallic,
             reflectance: self.reflectance,

examples/3d/bloom_3d.rs

@@ -42,15 +42,15 @@ fn setup_scene(
     ));
 
     let material_emissive1 = materials.add(StandardMaterial {
-        emissive: Color::rgb_linear(5000.0, 500.0, 50.0), // 4. Put something bright in a dark environment to see the effect
+        emissive: Color::rgb_linear(13.99, 5.32, 2.0), // 4. Put something bright in a dark environment to see the effect
         ..default()
     });
     let material_emissive2 = materials.add(StandardMaterial {
-        emissive: Color::rgb_linear(50.0, 5000.0, 500.0),
+        emissive: Color::rgb_linear(2.0, 13.99, 5.32),
         ..default()
     });
     let material_emissive3 = materials.add(StandardMaterial {
-        emissive: Color::rgb_linear(500.0, 50.0, 5000.0),
+        emissive: Color::rgb_linear(5.32, 2.0, 13.99),
         ..default()
     });
     let material_non_emissive = materials.add(StandardMaterial {

examples/3d/lighting.rs

@@ -138,7 +138,7 @@ fn setup(
                 })),
                 material: materials.add(StandardMaterial {
                     base_color: Color::RED,
-                    emissive: Color::rgba_linear(100.0, 0.0, 0.0, 0.0),
+                    emissive: Color::rgba_linear(7.13, 0.0, 0.0, 0.0),
                     ..default()
                 }),
                 ..default()
@@ -170,7 +170,7 @@ fn setup(
                 })),
                 material: materials.add(StandardMaterial {
                     base_color: Color::GREEN,
-                    emissive: Color::rgba_linear(0.0, 100.0, 0.0, 0.0),
+                    emissive: Color::rgba_linear(0.0, 7.13, 0.0, 0.0),
                     ..default()
                 }),
                 ..default()
@@ -198,7 +198,7 @@ fn setup(
                 })),
                 material: materials.add(StandardMaterial {
                     base_color: Color::BLUE,
-                    emissive: Color::rgba_linear(0.0, 0.0, 100.0, 0.0),
+                    emissive: Color::rgba_linear(0.0, 0.0, 7.13, 0.0),
                     ..default()
                 }),
                 ..default()

examples/3d/spotlight.rs

@@ -105,7 +105,7 @@ fn setup(
                             })),
                             material: materials.add(StandardMaterial {
                                 base_color: Color::MAROON,
-                                emissive: Color::rgba_linear(0.125, 0.0, 0.0, 0.0),
+                                emissive: Color::rgba_linear(0.369, 0.0, 0.0, 0.0),
                                 ..default()
                             }),
                             ..default()