HDRenderPipeline: Optimize SSS screen pass

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Resources/CombineSubsurfaceScattering.shader

 
                 float3 unused;
 
+                // Note: When we are in this SubsurfaceScattering shader we know that we are a SSS material. This shader is strongly coupled with the deferred Lit.shader.
+                // We can use the material classification facility to help the compiler to know we use SSS material and optimize the code (and don't require to read gbuffer with materialId).
+                uint featureFlags = MATERIALFEATUREFLAGS_LIT_SSS;
+
-                DECODE_FROM_GBUFFER(gbuffer, 0xFFFFFFFF, bsdfData, unused);
+                DECODE_FROM_GBUFFER(gbuffer, featureFlags, bsdfData, unused);
 
                 int    profileID   = bsdfData.subsurfaceProfile;
                 float  distScale   = bsdfData.subsurfaceRadius;