Add preliminary ambient probe support

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDStringConstants.cs

         public static readonly int _Source4 = Shader.PropertyToID("_Source4");
         public static readonly int _Result1 = Shader.PropertyToID("_Result1");
 
+        public static readonly int _AmbientProbe               = Shader.PropertyToID("_AmbientProbe");
         public static readonly int _GlobalFog_Extinction       = Shader.PropertyToID("_GlobalFog_Extinction");
         public static readonly int _GlobalFog_Scattering       = Shader.PropertyToID("_GlobalFog_Scattering");
         public static readonly int _GlobalFog_Asymmetry        = Shader.PropertyToID("_GlobalFog_Asymmetry");

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumetricLighting.compute

         float phase = IsotropicPhaseFunction();
     #endif
 
+        // Sample the light probe.
+        float3 probeRadiance = _AmbientProbe.rgb * (4 * PI) * TransmittanceIntegralHomogeneousMedium(extinction, dt);
+
-        totalRadiance += (transmittance * phase) * scattering * blendedRadiance;
+        totalRadiance += (transmittance * phase) * scattering * (blendedRadiance + probeRadiance);
 
         // Compute the optical depth up to the center of the interval.
         opticalDepth += 0.5 * extinction * dt;

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/VolumetricLighting.cs

         return depthParams;
     }
 
+    Vector4 GetAmbientLightProbe()
+    {
+        Vector4 probe = new Vector4();
+
+        SphericalHarmonicsL2 probeSH = RenderSettings.ambientProbe;
+        probe.x = probeSH[0, 0];
+        probe.y = probeSH[1, 0];
+        probe.z = probeSH[2, 0];
+
+        return probe;
+    }
+
     public void PushGlobalParams(HDCamera camera, CommandBuffer cmd)
     {
         if (preset == VolumetricLightingPreset.Off) return;
         cmd.SetGlobalVector( HDShaderIDs._VBufferDepthEncodingParams, ComputeLogarithmicDepthEncodingParams(m_VBufferNearPlane, m_VBufferFarPlane, k_LogScale));
         cmd.SetGlobalVector( HDShaderIDs._VBufferDepthDecodingParams, ComputeLogarithmicDepthDecodingParams(m_VBufferNearPlane, m_VBufferFarPlane, k_LogScale));
         cmd.SetGlobalTexture(HDShaderIDs._VBufferLighting,            vBuffer.GetLightingIntegralBuffer());
+        cmd.SetGlobalVector( HDShaderIDs._AmbientProbe,               GetAmbientLightProbe());
     }
 
     // Ref: https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres

ScriptableRenderPipeline/HDRenderPipeline/HDRP/ShaderVariables.hlsl

     float2 _TaaFrameRotation; // {x = sin(_TaaFrameIndex * PI/2), y = cos(_TaaFrameIndex * PI/2), z = unused}
     uint   _TaaFrameIndex;    // [0, 7]
     // Volumetric lighting.
+    float4 _AmbientProbe;
     float  _GlobalFog_Asymmetry;
     float3 _GlobalFog_Scattering;
     float  _GlobalFog_Extinction;