HDRenderPipeline: Fix issue with SurfaceGradient + update flip/mirror normal

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitData.hlsl

 {
     float3 vertexNormalWS = input.worldToTangent[2];
 
-    layerTexCoord.vertexTangentWS0 = input.worldToTangent[1];
-    layerTexCoord.vertexBitangentWS0 = input.worldToTangent[2];
+    layerTexCoord.vertexTangentWS0 = input.worldToTangent[0];
+    layerTexCoord.vertexBitangentWS0 = input.worldToTangent[1];
 
     // TODO: We should use relative camera position here - This will be automatic when we will move to camera relative space.
     float3 dPdx = ddx_fine(input.positionWS);
 
 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
-    ApplyDoubleSidedFlip(input); // Apply double sided flip on the vertex normal
+    ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
 
     LayerTexCoord layerTexCoord;
     ZERO_INITIALIZE(LayerTexCoord, layerTexCoord);
     // so it allow us to correctly deal with detail normal map and optimize the code for the layered shaders
     float3 normalTS;
     float alpha = GetSurfaceData(input, layerTexCoord, surfaceData, normalTS);
-    ApplyDoubleSidedMirror(input, normalTS); // Apply double sided mirror on the final normalTS
     GetNormalAndTangentWS(input, V, normalTS, surfaceData.normalWS, surfaceData.tangentWS);
     // Done one time for all layered - cumulate with spec occ alpha for now
     surfaceData.specularOcclusion *= GetHorizonOcclusion(V, surfaceData.normalWS, input.worldToTangent[2].xyz, _HorizonFade);
 
 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
-    ApplyDoubleSidedFlip(input); // Apply double sided flip on the vertex normal
+    ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
 
     LayerTexCoord layerTexCoord;
     ZERO_INITIALIZE(LayerTexCoord, layerTexCoord);
     surfaceData.coatPerceptualSmoothness = 1.0;
     surfaceData.specularColor = float3(0.0, 0.0, 0.0);
 
-    ApplyDoubleSidedMirror(input, normalTS); // Apply double sided mirror on the final normalTS
     GetNormalAndTangentWS(input, V, normalTS, surfaceData.normalWS, surfaceData.tangentWS);
     // Done one time for all layered - cumulate with spec occ alpha for now
     surfaceData.specularOcclusion = SURFACEDATA_BLEND_SCALAR(surfaceData, specularOcclusion, weights);

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/MaterialUtilities.hlsl

 #endif
 }
 
-// To flip in case of double sided, we must flip the vertex normal and this will apply to the whole process either in surface gradient or not.
-// As here we are in the function call GetSurfaceAndBuiltinData(), the tangent space is already built, so we need to flip both normal and bitangent.
+// Flipping or mirroring a normal can be done directly on the tangent space. This has the benefit to apply to the whole process either in surface gradient or not.
-void ApplyDoubleSidedFlip(inout FragInputs input)
+void ApplyDoubleSidedFlipOrMirror(inout FragInputs input)
-    float flipSign = input.isFrontFace ? 1.0 : _DoubleSidedConstants.x; // TOCHECK :  GetOddNegativeScale() is not necessary here as it is apply for tangent space creation.
-    input.worldToTangent[1] = flipSign * input.worldToTangent[1]; // bitangent
-    input.worldToTangent[2] = flipSign * input.worldToTangent[2]; // normal
+    // To get a flipped normal with the tangent space, we must flip bitangent (because it is construct from the normal) and normal
+    // To get a mirror normal with the tangent space, we only need to flip the normal and not the tangent
+    float2 flipSign = input.isFrontFace ? float2(1.0, 1.0) : _DoubleSidedConstants.yz; // TOCHECK :  GetOddNegativeScale() is not necessary here as it is apply for tangent space creation.
+    input.worldToTangent[1] = flipSign.x * input.worldToTangent[1]; // bitangent    
+    input.worldToTangent[2] = flipSign.y * input.worldToTangent[2]; // normal
-    #endif
-#endif
-}
-
-// To mirror a normal: in ws reflect around the vertex normal / in tangent space apply minus on the z component.
-// For surface gradient it is sufficient to take the opposite of the surface gradient.
-void ApplyDoubleSidedMirror(FragInputs input, inout float3 normalTS)
-{
-#ifdef _DOUBLESIDED_ON
-    // _DoubleSidedConstants is float3(-1, -1, -1) in flip mode and float3(1, 1, -1) in mirror mode
-    float flipSign = input.isFrontFace ? 1.0 : -_DoubleSidedConstants.x; // TOCHECK :  GetOddNegativeScale() is not necessary here as it is apply for tangent space creation.
-    #ifdef SURFACE_GRADIENT
-    normalTS = flipSign * normalTS;
-    #else
-    normalTS.z *= flipSign;
     #endif
 #endif
 }