Fix directional light and shadow mask transition

The shadow mask transition now follow the shadow cascade transition

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightDefinition.cs

         public Vector3 up;      // Rescaled by (2 / shapeHeight)
         public float diffuseScale;
 
-        public Vector2 fadeDistanceScaleAndBias; // Use with ShadowMask feature
-        public float unused0;
+        public float volumetricDimmer;
-
-        public float volumetricDimmer; // TODO: improve the cache locality
     };
 
     [GenerateHLSL]

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightDefinition.cs.hlsl

     float specularScale;
     float3 up;
     float diffuseScale;
-    float2 fadeDistanceScaleAndBias;
-    float unused0;
+    float volumetricDimmer;
-    float volumetricDimmer;
 };
 
 // Generated from UnityEngine.Experimental.Rendering.HDPipeline.LightData
 {
 	return value.diffuseScale;
 }
-float2 GetFadeDistanceScaleAndBias(DirectionalLightData value)
-{
-	return value.fadeDistanceScaleAndBias;
-}
-float GetUnused0(DirectionalLightData value)
+float GetVolumetricDimmer(DirectionalLightData value)
-	return value.unused0;
+	return value.volumetricDimmer;
 }
 int GetDynamicShadowCasterOnly(DirectionalLightData value)
 {
 {
 	return value.shadowMaskSelector;
-}
-float GetVolumetricDimmer(DirectionalLightData value)
-{
-	return value.volumetricDimmer;
 }
 
 //

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightEvaluation.hlsl

 #endif
 
 #ifdef SHADOWS_SHADOWMASK
-        float fade = saturate(posInput.linearDepth * lightData.fadeDistanceScaleAndBias.x + lightData.fadeDistanceScaleAndBias.y);
+
+        // TODO: Optimize this code! Currently it is a bit like brute force to get the last transistion and fade to shadow mask, but there is
+        // certainly more efficient to do
+        // We reuse the transition from the cascade system to fade between shadow mask at max distance
+        uint  payloadOffset;
+        real  fade;
+        int cascadeCount;
+        int shadowSplitIndex = EvalShadow_GetSplitIndex(lightLoopContext.shadowContext, lightData.shadowIndex, positionWS, payloadOffset, fade, cascadeCount);
+        // we have a fade caclulation for each cascade but we must lerp with shadow mask only for the last one
+        fade = ((shadowSplitIndex + 1) == cascadeCount || shadowSplitIndex == -1.0) ? fade : 0.0;
-        shadow = lerp(shadow, shadowMask, fade); // Caution to lerp parameter: fade is the reverse of shadowDimmer
+        // In the transition code (both dithering and blend) we use shadow = lerp( shadow, 1.0, fade ) for last transition
+        // mean if we expend the code we have (shadow * (1 - fade) + fade). Here to make transition with shadow mask
+        // we will remove fade and add fade * shadowMask which mean we do a lerp with shadow mask
+        shadow = shadow - fade + fade * shadowMask;
 
         // Note: There is no shadowDimmer when there is no shadow mask
 #endif

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightLoop/LightLoop.cs

                 m_CurrentSunLightShadowIndex = shadowIdx;
             }
 
-            float scale;
-            float bias;
-            GetScaleAndBiasForLinearDistanceFade(m_maxShadowDistance, out scale, out bias);
-            directionalLightData.fadeDistanceScaleAndBias = new Vector2(scale, bias);
             directionalLightData.shadowMaskSelector = Vector4.zero;
 
             if (IsBakedShadowMaskLight(light.light))