Add mesh filter and mesh renderer for emissive mesh on lights

com.unity.render-pipelines.high-definition/HDRP/Editor/Lighting/HDLightEditor.Styles.cs

             public readonly GUIContent shapeWidthBox = new GUIContent("Size X", "");
             public readonly GUIContent shapeHeightBox = new GUIContent("Size Y", "");
             public readonly GUIContent applyRangeAttenuation = new GUIContent("Apply Range Attenuation", "Allows disabling range attenuation. This is useful indoor (like a room) to avoid having to setup a large range for a light to get correct inverse square attenuation that may leak out of the indoor");
+            public readonly GUIContent showEmissiveMesh = new GUIContent("Show Emissive Mesh", "Generate an emissive mesh using the size and intensity of the light");
 
             public readonly GUIContent shape = new GUIContent("Type", "Specifies the current type of light. Possible types are Directional, Spot, Point, Rectangle and Line lights.");
             public readonly GUIContent[] shapeNames;

com.unity.render-pipelines.high-definition/HDRP/Editor/Lighting/HDLightEditor.cs

             public SerializedProperty maxSmoothness;
             public SerializedProperty applyRangeAttenuation;
             public SerializedProperty volumetricDimmer;
+            public SerializedProperty showEmissiveMesh;
 
             // Editor stuff
             public SerializedProperty useOldInspector;
         // Used for UI only; the processing code must use LightTypeExtent and LightType
         LightShape m_LightShape;
 
+        HDAdditionalLightData[]     m_AdditionalLightDatas;
+        AdditionalShadowData[]      m_AdditionalShaodowDatas;
+
-            var lightData = CoreEditorUtils.GetAdditionalData<HDAdditionalLightData>(targets, HDAdditionalLightData.InitDefaultHDAdditionalLightData);
-            var shadowData = CoreEditorUtils.GetAdditionalData<AdditionalShadowData>(targets, HDAdditionalShadowData.InitDefaultHDAdditionalShadowData);
-            m_SerializedAdditionalLightData = new SerializedObject(lightData);
-            m_SerializedAdditionalShadowData = new SerializedObject(shadowData);
+            m_AdditionalLightDatas = CoreEditorUtils.GetAdditionalData<HDAdditionalLightData>(targets, HDAdditionalLightData.InitDefaultHDAdditionalLightData);
+            m_AdditionalShaodowDatas = CoreEditorUtils.GetAdditionalData<AdditionalShadowData>(targets, HDAdditionalShadowData.InitDefaultHDAdditionalShadowData);
+            m_SerializedAdditionalLightData = new SerializedObject(m_AdditionalLightDatas);
+            m_SerializedAdditionalShadowData = new SerializedObject(m_AdditionalShaodowDatas);
 
             using (var o = new PropertyFetcher<HDAdditionalLightData>(m_SerializedAdditionalLightData))
                 m_AdditionalLightData = new SerializedLightData
                     spotInnerPercent = o.Find(x => x.m_InnerSpotPercent),
                     lightDimmer = o.Find(x => x.lightDimmer),
                     volumetricDimmer = o.Find(x => x.volumetricDimmer),
+                    showEmissiveMesh = o.Find(x => x.showEmissiveMesh),
                     fadeDistance = o.Find(x => x.fadeDistance),
                     affectDiffuse = o.Find(x => x.affectDiffuse),
                     affectSpecular = o.Find(x => x.affectSpecular),
 
             if (EditorGUI.EndChangeCheck())
             {
+                UpdateEmissiveMesh();
+        void UpdateEmissiveMesh()
+        {
+            foreach (var lightData in m_AdditionalLightDatas)
+            {
+                Debug.Log("heavy operation");
+                GameObject    lightGameObject = lightData.gameObject;
+                MeshRenderer  emissiveMeshRenderer = lightData.GetComponent<MeshRenderer>();
+                MeshFilter    emissiveMeshFilter = lightData.GetComponent<MeshFilter>();
+
+                // Ensure that the emissive mesh components are here
+                if (lightData.showEmissiveMesh)
+                {
+                    if (emissiveMeshRenderer == null)
+                        emissiveMeshRenderer = lightGameObject.AddComponent<MeshRenderer>();
+                    if (emissiveMeshFilter == null)
+                        emissiveMeshFilter = lightGameObject.AddComponent<MeshFilter>();
+                }
+                else // Or remove them if the option is disabled
+                {
+                    if (emissiveMeshRenderer != null)
+                        DestroyImmediate(emissiveMeshRenderer);
+                    if (emissiveMeshFilter != null)
+                        DestroyImmediate(emissiveMeshFilter);
+                }
+
+                if (lightData.showEmissiveMesh)
+                {
+                    // Update Mesh emissive value
+                }
+            }
+        }
+
         // Caution: this function must match the one in HDAdditionalLightData.ConvertPhysicalLightIntensityToLightIntensity - any change need to be replicated
         void UpdateLightIntensity()
         {
                     EditorGUILayout.PropertyField(m_AdditionalLightData.shapeHeight, s_Styles.cookieSizeY);
                     EditorGUI.indentLevel--;
                 }
+            }
+            
+            // Emissive mesh for area light only
+            if (m_LightShape == LightShape.Rectangle)
+            {
+                EditorGUI.BeginChangeCheck();
+                EditorGUILayout.PropertyField(m_AdditionalLightData.showEmissiveMesh);
+                if (EditorGUI.EndChangeCheck())
+                    UpdateEmissiveMesh();
             }
 
             if (m_AdditionalLightData.showAdditionalSettings.boolValue)

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

         public bool featuresFoldout = true;
         public bool showAdditionalSettings = false;
 
+        public bool showEmissiveMesh = false;
+
 #if UNITY_EDITOR
 
         private void DrawGizmos(bool selected)