Fix additive blending in the deferred shader

Assets/ScriptableRenderLoop/HDRenderPipeline/Lighting/Resources/Deferred.shader

 
             ZWrite Off
             ZTest  Always
-            Blend [_SrcBlend][_DstBlend]
+            Blend [_SrcBlend] [_DstBlend], One Zero
 
             HLSLPROGRAM
             #pragma target 4.5

Assets/ScriptableRenderLoop/HDRenderPipeline/Lighting/TilePass/TilePass.cs

                 m_DeferredIndirectMaterialSRT.DisableKeyword("OUTPUT_SPLIT_LIGHTING");
                 m_DeferredIndirectMaterialSRT.SetInt("_StencilRef", (int)StencilBits.None);
                 m_DeferredIndirectMaterialSRT.SetInt("_SrcBlend", (int)BlendMode.One);
-                m_DeferredIndirectMaterialSRT.SetInt("_DstBlend", (int)BlendMode.One); // Additive
+                m_DeferredIndirectMaterialSRT.SetInt("_DstBlend", (int)BlendMode.One); // Additive color & alpha source
 
                 m_DeferredIndirectMaterialMRT = Utilities.CreateEngineMaterial("Hidden/HDRenderPipeline/Deferred");
                 Utilities.SelectKeyword(m_DeferredIndirectMaterialMRT, tileKeywords, 1);
                 m_DeferredIndirectMaterialMRT.SetInt("_SrcBlend", (int)BlendMode.One);
-                m_DeferredIndirectMaterialMRT.SetInt("_DstBlend", (int)BlendMode.One); // Additive
+                m_DeferredIndirectMaterialMRT.SetInt("_DstBlend", (int)BlendMode.One); // Additive color & alpha source
 
                 m_DeferredAllMaterialSRT = Utilities.CreateEngineMaterial("Hidden/HDRenderPipeline/Deferred");
                 Utilities.SelectKeyword(m_DeferredAllMaterialSRT, tileKeywords, 2);