[Port] [2022.3] [HDRP] Fix sun flicker where the sun is close to clou…

…d boundaries.

Jira: [UUM-70628](https://jira.unity3d.com/browse/UUM-70628)
Parent PR: [#48000](https://github.cds.internal.unity3d.com/unity/unity/pull/48000)

As reported in JIRA, the sun flicker problem occurs when the directional light is close to cloud boundaries. This can only be reproduced if `perceptual blending` is enabled on volumetric clouds. The transmittance of each pixel changes every frame due to ray marching noise and temporal reprojection. When perceptual blending is enabled, it divides the transmittance by the current luminance of the target pixel, so it reduces the RGB value dramatically when the luminance is really high.

![image](https://media.github.cds.internal.unity3d.com/user/3842/files/c2c067db-d228-4500-b28d-4f9d0295c004)

When the `perceptual blending` is disabled, we simply use the original input transmittance, so that neighboring pixels have slightly different values. However, we reduce the transmittance by luminance when `perceptual blending` is enabled. This causes a huge pixel value change for neighboring pixels at the cloud boundaries, so the sun flickers.
![image](https://media.github.cds.internal.unity3d.com/user/3842/files/06816951-ac8b-4523-a0e6-36db227fc146)

By softening the transmittance attenuation curve with a `pow(transmittance, 6)` clamp, we can prevent sun flicker and slightly improve the perceptual blending where the sun is behind the clouds.
![image](https://media.github.cds.internal.unity3d.com/user/3842/files/cce785be-a336-48b9-871b-aa53443c58b2)


| Before | After |
| -- | -- |
| ![image](https://media.github.cds.internal.unity3d.com/user/3842/files/594c9648-dd7e-4fa0-aa72-4f4cd3643ac3)|![image](https://media.github.cds.internal.unity3d.com/user/3842/files/cb52d81c-82c6-4b7a-bdff-d8dda24d7bf6)|

Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/VolumetricClouds/VolumetricCloudsUtilities.hlsl

@@ -819,8 +819,12 @@ float EvaluateFinalTransmittance(float3 color, float transmittance)
     // reverse the tone mapping
     resultLuminance = resultLuminance / (1.0 - resultLuminance);
 
+    // By softening the transmittance attenuation curve for pixels adjacent to cloud boundaries when the luminance is super high,  
+    // We can prevent sun flicker and improve perceptual blending. (https://www.desmos.com/calculator/vmly6erwdo)
+    float finalTransmittance = max(resultLuminance / luminance, pow(transmittance, 6));
+
     // This approach only makes sense if the color is not black
-    return luminance > 0.0 ? lerp(transmittance, resultLuminance / luminance, _ImprovedTransmittanceBlend) : transmittance;
+    return luminance > 0.0 ? lerp(transmittance, finalTransmittance, _ImprovedTransmittanceBlend) : transmittance;
 }
 
 #endif // VOLUMETRIC_CLOUD_UTILITIES_H