Fix MSAA resolve of stencil buffer (#6251)

* Fix resolve

* changelog

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

Author: FrancescoC-unity

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

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed rendertarget ColorMask in Forward with virtual texturing and transparent motion vectors.
 - Fixed light unit conversion after changing mid gray value.
 - Fixed Focus distance in path traced depth of field now takes into account the focus mode setting (volume vs camera).
+- Fixed stencil buffer resolve when MSAA is enabled so that OR operator is used instead of picking the last sample.
 
 ### Changed
 - Maximum light count per fine prunned tile (opaque deferred) is now 63 instead of 23.

com.unity.render-pipelines.high-definition/Runtime/ShaderLibrary/ResolveStencilBuffer.compute

@@ -47,7 +47,7 @@ void KERNEL_NAME(uint3 groupId          : SV_GroupID,
 {
     UNITY_XR_ASSIGN_VIEW_INDEX(dispatchThreadID.z);
 
-    // The best shot at resolving is being overly conservative, hence the OR operator. This is by nature inaccurate.
+    // The best shot at resolving is being overly conservative, hence the OR operator. This is by nature inaccurate, but there is no way to blend MSAA sub-samples properly and we need to pick the lesser evil.
     uint resolvedStencil = 0;
 
     if (dispatchThreadID.x < (uint)_ScreenSize.x && dispatchThreadID.y < (uint)_ScreenSize.y)
@@ -61,7 +61,7 @@ void KERNEL_NAME(uint3 groupId          : SV_GroupID,
 #else
             sampledStencil = LOAD_TEXTURE2D_X_MSAA(_StencilTexture, dispatchThreadID.xy, i);
 #endif
-            resolvedStencil = GetStencilValue(sampledStencil);
+            resolvedStencil |= GetStencilValue(sampledStencil); // In not MSAA cases the | is the same as assigning given that NUM_SAMPLES is 1
         }
     }
 #ifdef RESOLVE