Add shadowcaster blur support to shaders

Introduces a _BlurPixels property and related parameters to BlendinShader and LitParticles shaders, enabling blurred shadowcaster sampling. Updates Shadowcaster.cginc to support blurred texture sampling using tex2Dgrad when blur is enabled. Also adjusts light intensity calculation in LightStrength.hlsl to remove NdotL multiplication for consistency with new shadow handling.

Shader/Includes/Shadowcaster.cginc

@@ -1,14 +1,15 @@
 #ifndef SHADOWCASTER_PLANE
 #define SHADOWCASTER_PLANE
 
-#include "UnityCG.cginc"   // for tex2Dlod, etc.
+#include "UnityCG.cginc" // tex2D, tex2Dgrad
 
 static const float WS_EPS = 1e-5;
 
 inline float4 SampleShadowcasterPlaneWS_Basis(
     float3 A, float3 B,
     float3 P0, float3 Uinv, float3 Vinv, float3 N,
-    sampler2D tex, float4 OutsideColor, float4 ShadowColor)
+    sampler2D tex, float4 OutsideColor, float4 ShadowColor,
+    float blurPixels, float2 texelSize)
 {
     float3 d  = B - A;
     float  dn = dot(N, d);
@@ -16,6 +17,7 @@ inline float4 SampleShadowcasterPlaneWS_Basis(
 
     float  t  = dot(N, P0 - A) / dn;
     if (t < 0.0 || t > 1.0) return OutsideColor;
+
     float3 hit = A + d * t;
     float3 r   = hit - P0;
 
@@ -24,9 +26,21 @@ inline float4 SampleShadowcasterPlaneWS_Basis(
     float v = dot(r, Vinv);
     if (abs(u) > 0.5 || abs(v) > 0.5) return OutsideColor;
 
-    float4 returnColor = tex2D(tex, float2(u + 0.5, v + 0.5)) * ShadowColor;
-    returnColor = float4(returnColor.rgb * (1 - returnColor.a), 1);
-    return returnColor;
+    float2 uv = float2(u + 0.5, v + 0.5);
+
+    // If blur is tiny, do the normal one-tap
+    if (blurPixels <= 0.001)
+    {
+        float4 col = tex2D(tex, uv) * ShadowColor;
+        return float4(col.rgb * (1 - col.a), 1);
+    }
+
+    // Inflate gradients so the sampler picks a higher mip (cheap blur).
+    float2 g = texelSize * blurPixels;
+    float4 blurred = tex2Dgrad(tex, uv, float2(g.x, 0), float2(0, g.y));
+
+    float4 outCol = blurred * ShadowColor;
+    return float4(outCol.rgb * (1 - outCol.a), 1);
 }
 
 #endif