Add analytical transmittance integration

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

     return exp(-opticalDepth);
 }
 
+float TransmittanceIntegralOverHomogeneousInterval(float extinction, float start, float end)
+{
+    return (exp(-extinction * start) - exp(-extinction * end)) / extinction;
+}
+
 float3 OpticalDepthHomogeneous(float3 extinction, float intervalLength)
 {
     return extinction * intervalLength;
 {
     return exp(-opticalDepth);
+}
+
+float3 TransmittanceIntegralOverHomogeneousInterval(float3 extinction, float start, float end)
+{
+    return (exp(-extinction * start) - exp(-extinction * end)) / extinction;
 }
 
 float IsotropicPhaseFunction()

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

     float du = rcp(numSteps);
     float u0 = 0.25 * du + 0.5 * du * zeta;
     float dt = du * ray.maxLength;
+    float t0 = 0;
-        float u = u0 + s * du;          // [0, 1]
-        float t = u * ray.maxLength;    // [0, ray.maxLength]
+        float u  = u0 + s * du;          // [0, 1]
+        float t  = u * ray.maxLength;    // [0, ray.maxLength]
+        float t1 = t + 0.5 * dt;
+
+        float T = TransmittanceIntegralOverHomogeneousInterval(extinction, t0, t1);
+
+        [branch] if (T < 0.0001) break;
 
         float3 positionWS = ray.originWS + t * ray.directionWS;
 
             }
         }
 
-        radiance += sampleRadiance * (Transmittance(OpticalDepthHomogeneous(extinction, t)) * dt);
+        radiance += sampleRadiance * T;
+        t0 = t1;
     }
 #ifdef USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
     float3 phaseConstant = scattering * HenyeyGreensteinPhasePartConstant(asymmetry);