Fix issue with Uber Post process shader when alpha is enabled (#422)

* Fix issue with alpha being over 1 hence, leading to the final color being boosted due to alpha handling

* Changelog

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: FrancescoC-unity

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -583,6 +583,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed issue with reflection probes in realtime time mode with OnEnable baking having wrong lighting with sky set to dynamic (case 1238047).
 - Fixed transparent motion vectors not working when in MSAA.
 - Fix error when removing DecalProjector from component contextual menu (case 1243960)
+- Fixed issue with post process when running in RGBA16 and an object with additive blending is in the scene.
 
 ### Changed
 - Improve MIP selection for decals on Transparents

com.unity.render-pipelines.high-definition/Runtime/PostProcessing/Shaders/UberPost.compute

@@ -218,7 +218,8 @@ void Uber(uint3 dispatchThreadId : SV_DispatchThreadID)
     // Alpha mask
 #ifdef ENABLE_ALPHA
         // Post processing is not applied on pixels with zero alpha
-        color.xyz = lerp(inputColor.xyz, color.xyz, inputColor.a);
+        // Saturate is necessary to avoid issues when additive blending pushes the alpha over 1.
+        color.xyz = lerp(inputColor.xyz, color.xyz, saturate(inputColor.a));
 #endif
 
     // Done