Fix emissive mesh layer mask behaviour (#1062)

* Update emissive game object layer mask on validation

* changelog

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

Author: FrancescoC-unity

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

@@ -710,6 +710,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed Sorting Priority not displayed correctly in the DrawRenderers custom pass UI.
 - Fixed glitch in Project settings window when selecting diffusion profiles in material section (case 1253090)
 - Fixed issue with light layers bigger than 8 (and above the supported range). 
+- Fixed issue with culling layer mask of area light's emissive mesh 
 
 ### Changed
 - Improve MIP selection for decals on Transparents

com.unity.render-pipelines.high-definition/Runtime/Lighting/Light/HDAdditionalLightData.cs

@@ -2507,6 +2507,11 @@ void OnValidate()
 
             RefreshCachedShadow();
 
+            if (emissiveMeshRenderer != null && !emissiveMeshRenderer.Equals(null))
+            {
+                emissiveMeshRenderer.gameObject.layer = m_AreaLightEmissiveMeshLayer;
+            }
+
 #if UNITY_EDITOR
             // If modification are due to change on prefab asset, we want to have prefab instances to self-update, but we cannot check in OnValidate if this is part of
             // prefab instance. So we delay the check on next update (and before teh LateUpdate logic)