add a table for enabling writes to gbuffer render targets

com.unity.render-pipelines.high-definition/HDRP/Material/GBufferManager.cs

         public override void CreateBuffers()
         {
             RenderTextureFormat[] rtFormat;
-            bool[] sRGBFlags;            
-            m_DeferredMaterial.GetMaterialGBufferDescription(m_asset, out rtFormat, out sRGBFlags, out m_GBufferUsage);
+            bool[] sRGBFlags;
+            bool[] enableWrite;
+            m_DeferredMaterial.GetMaterialGBufferDescription(m_asset, out rtFormat, out sRGBFlags, out m_GBufferUsage, out enableWrite);
-                m_RTs[gbufferIndex] = RTHandles.Alloc(Vector2.one, colorFormat: rtFormat[gbufferIndex], sRGB: sRGBFlags[gbufferIndex], filterMode: FilterMode.Point, name: string.Format("GBuffer{0}", gbufferIndex), enableRandomWrite: gbufferIndex == 1); // normal buffer is used as RWTexture to composite decals in forward
+                m_RTs[gbufferIndex] = RTHandles.Alloc(Vector2.one, colorFormat: rtFormat[gbufferIndex], sRGB: sRGBFlags[gbufferIndex], filterMode: FilterMode.Point, name: string.Format("GBuffer{0}", gbufferIndex), enableRandomWrite: enableWrite[gbufferIndex]); 
                 m_RTIDs[gbufferIndex] = m_RTs[gbufferIndex].nameID;
                 m_TextureShaderIDs[gbufferIndex] = HDShaderIDs._GBufferTexture[gbufferIndex];
 

com.unity.render-pipelines.high-definition/HDRP/Material/Lit/Lit.cs

             return gBufferCount;
         }
 
-        public override void GetMaterialGBufferDescription(HDRenderPipelineAsset asset, out RenderTextureFormat[] RTFormat, out bool[] sRGBFlag, out GBufferUsage[] gBufferUsage)
+        public override void GetMaterialGBufferDescription(HDRenderPipelineAsset asset, out RenderTextureFormat[] RTFormat, out bool[] sRGBFlag, out GBufferUsage[] gBufferUsage, out bool[] enableWrite)
         {
             int gBufferCount;
             bool supportShadowMask;
             RTFormat = new RenderTextureFormat[gBufferCount];
             sRGBFlag = new bool[gBufferCount];
-            gBufferUsage = new GBufferUsage[gBufferCount];            
+            gBufferUsage = new GBufferUsage[gBufferCount];
+            enableWrite = new bool[gBufferCount];
+            enableWrite[0] = false;
+            enableWrite[1] = true;                    // normal buffer is used as RWTexture to composite decals in forward
+            enableWrite[2] = false;
+            enableWrite[3] = false;
 
             int index = 4;
 

com.unity.render-pipelines.high-definition/HDRP/Material/RenderPipelineMaterial.cs

         // GBuffer management
         public virtual bool IsDefferedMaterial() { return false; }
         public virtual int GetMaterialGBufferCount(HDRenderPipelineAsset asset) { return 0; }
-        public virtual void GetMaterialGBufferDescription(HDRenderPipelineAsset asset, out RenderTextureFormat[] RTFormat, out bool[] sRGBFlag, out GBufferUsage[] gBufferUsage)
+        public virtual void GetMaterialGBufferDescription(HDRenderPipelineAsset asset, out RenderTextureFormat[] RTFormat, out bool[] sRGBFlag, out GBufferUsage[] gBufferUsage, out bool[] enableWrite)
-    }
+            enableWrite = null;
+        }
 
         // Regular interface
         public virtual void Build(HDRenderPipelineAsset hdAsset) {}