[Unity 6][URP] Fixing an issue where Shadow Near Plane incorrectly culls casters (UUM-63997)

Fixes UUM-63997.

Author: ellioman

Packages/com.unity.render-pipelines.universal/Shaders/ShadowCasterPass.hlsl

@@ -30,6 +30,7 @@ struct Varyings
     UNITY_VERTEX_INPUT_INSTANCE_ID
 };
 
+#define EPSILON 0.001
 float4 GetShadowPositionHClip(Attributes input)
 {
     float3 positionWS = TransformObjectToWorld(input.positionOS.xyz);
@@ -43,11 +44,12 @@ float4 GetShadowPositionHClip(Attributes input)
 
     float4 positionCS = TransformWorldToHClip(ApplyShadowBias(positionWS, normalWS, lightDirectionWS));
 
-#if UNITY_REVERSED_Z
-    positionCS.z = min(positionCS.z, UNITY_NEAR_CLIP_VALUE);
-#else
-    positionCS.z = max(positionCS.z, UNITY_NEAR_CLIP_VALUE);
-#endif
+    #if UNITY_REVERSED_Z
+        float clamped = min(positionCS.z, positionCS.w * UNITY_NEAR_CLIP_VALUE);
+    #else
+        float clamped = max(positionCS.z, positionCS.w * UNITY_NEAR_CLIP_VALUE);
+    #endif
+    positionCS.z = lerp(positionCS.z, clamped, saturate(_ShadowBias.y + EPSILON));
 
     return positionCS;
 }