Make the SSS LoD threshold configurable

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

 
                 // Use fewer samples for SS regions smaller than 5x5 pixels (rotated by 45 degrees).
                 [branch]
-                if (maxDistancePixels < 4)
+                if (maxDistancePixels < SSS_LOD_THRESHOLD)
                 {
                     #if SSS_DEBUG
                         return float4(0.5, 0.5, 0, 1);

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/SubsurfaceScatteringProfile.cs.hlsl

 #define SSS_NEUTRAL_PROFILE_ID (7)
 #define SSS_N_SAMPLES_NEAR_FIELD (55)
 #define SSS_N_SAMPLES_FAR_FIELD (21)
+#define SSS_LOD_THRESHOLD (4)
 #define SSS_TRSM_MODE_NONE (0)
 #define SSS_TRSM_MODE_THIN (1)