Avoid shading pixels behind rectangular lights

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

     diffuseLighting  = float3(0.0, 0.0, 0.0);
     specularLighting = float3(0.0, 0.0, 0.0);
 
-    // TODO: This could be precomputed.
-    float halfWidth  = lightData.size.x * 0.5;
-    float halfHeight = lightData.size.y * 0.5;
+    float3 unL = lightData.positionWS - positionWS;
-    float3 unL = lightData.positionWS - positionWS;
+    [branch]
+    if (dot(lightData.forward, unL) >= 0)
+    {
+        // The light is back-facing.
+        return;
+    }
+
+    // TODO: This could be precomputed.
+    float halfWidth  = lightData.size.x * 0.5;
+    float halfHeight = lightData.size.y * 0.5;
 
     // Define the dimensions of the attenuation volume.
     // TODO: This could be precomputed.