added depth bias to mesh decals

com.unity.render-pipelines.high-definition/HDRP/Editor/Material/Decal/DecalUI.cs

             public static GUIContent normalMapText = new GUIContent("Normal Map", "Normal Map (BC7/BC5/DXT5(nm))");
             public static GUIContent decalBlendText = new GUIContent("Global Opacity", "Whole decal Opacity");
             public static GUIContent AlbedoModeText = new GUIContent("Affect BaseColor", "Base color + Opacity, Opacity only");
+ 			public static GUIContent MeshDecalDepthBiasText = new GUIContent("Mesh decal depth bias", "prevents z-fighting");
 
             public static GUIContent[] maskMapText =
             {
         protected MaterialProperty maskmapSmoothness = new MaterialProperty();
         protected const string kMaskmapSmoothness = "_MaskmapSmoothness";
 
+        protected MaterialProperty decalMeshDepthBias = new MaterialProperty();
+        protected const string kDecalMeshDepthBias = "_DecalMeshDepthBias";
+
         protected MaterialEditor m_MaterialEditor;
 
         // This is call by the inspector
             maskmapMetal = FindProperty(kMaskmapMetal, props);
             maskmapAO = FindProperty(kMaskmapAO, props);
             maskmapSmoothness = FindProperty(kMaskmapSmoothness, props);
+            decalMeshDepthBias = FindProperty(kDecalMeshDepthBias, props);
             
             // always instanced
             SerializedProperty instancing = m_MaterialEditor.serializedObject.FindProperty("m_EnableInstancingVariants");
                     EditorGUI.indentLevel--;
                 }
                 m_MaterialEditor.ShaderProperty(decalBlend, Styles.decalBlendText);
+                m_MaterialEditor.ShaderProperty(decalMeshDepthBias, Styles.MeshDecalDepthBiasText);
                 EditorGUI.indentLevel--;
 
                 EditorGUILayout.HelpBox(

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/Decal.shader

 		[ToggleUI] _MaskmapMetal("_MaskmapMetal", Range(0.0, 1.0)) = 0.0
 		[ToggleUI] _MaskmapAO("_MaskmapAO", Range(0.0, 1.0)) = 0.0
 		[ToggleUI] _MaskmapSmoothness("_MaskmapSmoothness", Range(0.0, 1.0)) = 1.0
+		[HideInInspector] _DecalMeshDepthBias("_DecalMeshDepthBias", Float) = 0.0 
     }
 
     HLSLINCLUDE

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/DecalProperties.hlsl

 
 float _DecalBlend;
 float4 _BaseColor;
+float _DecalMeshDepthBias;
 
 #endif

com.unity.render-pipelines.high-definition/HDRP/ShaderPass/ShaderPassDBuffer.hlsl

 #include "VertMesh.hlsl"
 
 
+void MeshDecalsPositionZBias(inout VaryingsToPS input)
+{
+#if defined(UNITY_REVERSED_Z)
+	input.vmesh.positionCS.z -= _DecalMeshDepthBias * input.vmesh.positionCS.w;
+#else
+	input.vmesh.positionCS.z += _DecalMeshDepthBias * input.vmesh.positionCS.w;
+#endif
+}
+
+#if (SHADERPASS == SHADERPASS_DBUFFER_MESH)
+	MeshDecalsPositionZBias(varyingsType);
+#endif
     return PackVaryingsType(varyingsType);
 }