Fix PBR Accumulation for 10.6 (#5991)

* Fix PBR Accumulation for 10.6

* Add ChangeLog

Author: skhiat

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

@@ -154,6 +154,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed undo of some properties on light editor.
 - Fixed material Emission properties not begin animated when recording an animation (case 1328108).
 - Fixed issue with velocity rejection when using physically-based DoF
+- Fixed SSR PBR Accumulation algorithm, which was constantly cleared
 
 ### Changed
 - Reduced the maximal number of bounces for both RTGI and RTR (case 1318876).

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipeline.LightLoop.cs

@@ -542,8 +542,13 @@ TextureHandle RenderSSR(    RenderGraph         renderGraph,
                 if (!hdCamera.colorPyramidHistoryIsValid)
                 {
                     hdCamera.colorPyramidHistoryIsValid = true; // For the next frame...
+                    hdCamera.colorPyramidHistoryValidFrames = 0;
                     result = renderGraph.defaultResources.blackTextureXR;
                 }
+                else
+                {
+                    hdCamera.colorPyramidHistoryValidFrames++;
+                }
             }
 
             PushFullScreenDebugTexture(renderGraph, result, transparent ? FullScreenDebugMode.TransparentScreenSpaceReflections : FullScreenDebugMode.ScreenSpaceReflections);