Fix missing sRGB conversion for dithering non-HDR pipelines (#6707)

# Objective

- Fixes #6706 

Zoom in on the shadow in the following images:

## Current bevy/main

### HDR On - correct
![current-hdron](https://user-images.githubusercontent.com/2632925/202943151-ecad3cbe-a76e-46df-bac9-9e590a31a9f3.png)

### HDR Off - incorrect
![current-hdroff](https://user-images.githubusercontent.com/2632925/202943154-34e3f527-a00e-4546-931d-0691204cc6a4.png)

## This PR

### HDR On - correct
![new-hdron](https://user-images.githubusercontent.com/2632925/202943383-081990de-9a14-45bd-ac52-febcc4289079.png)

### HDR Off - corrected
![new-hdroff](https://user-images.githubusercontent.com/2632925/202943388-a3b05d79-a0f3-4b1e-b114-0a9f03efe351.png)

## Close-up comparison

### New
![Screenshot from 2022-11-20 17-46-46](https://user-images.githubusercontent.com/2632925/202943552-d45c3a48-841e-47a6-981f-776c5a9563f6.png)

### Old
![Screenshot from 2022-11-20 17-46-41](https://user-images.githubusercontent.com/2632925/202943562-555cb5a2-2b20-45f9-b250-89f2bc87af5f.png)

## Solution

- It turns out there was an outright missing sRGB conversion for dithering non-HDR cameras.
- I also tried using a precise sRGB conversion, but it had no apparent effect on the final image.

---

## Changelog

- Fix deband dithering intensity for non-HDR pipelines.

Author: aevyrie

crates/bevy_core_pipeline/src/tonemapping/tonemapping_shared.wgsl

@@ -34,4 +34,4 @@ fn screen_space_dither(frag_coord: vec2<f32>) -> vec3<f32> {
     var dither = vec3<f32>(dot(vec2<f32>(171.0, 231.0), frag_coord)).xxx;
     dither = fract(dither.rgb / vec3<f32>(103.0, 71.0, 97.0));
     return (dither - 0.5) / 255.0;
-}
+}

crates/bevy_pbr/src/render/pbr.wgsl

@@ -99,7 +99,13 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
         output_color = tone_mapping(output_color);
 #endif
 #ifdef DEBAND_DITHER
-        output_color = dither(output_color, in.frag_coord.xy);
+    var output_rgb = output_color.rgb;
+    output_rgb = pow(output_rgb, vec3<f32>(1.0 / 2.2));
+    output_rgb = output_rgb + screen_space_dither(in.frag_coord.xy);
+    // This conversion back to linear space is required because our output texture format is
+    // SRGB; the GPU will assume our output is linear and will apply an SRGB conversion.
+    output_rgb = pow(output_rgb, vec3<f32>(2.2));
+    output_color = vec4(output_rgb, output_color.a);
 #endif
     return output_color;
 }