Re-add sky support to volumetrics

ScriptableRenderPipeline/HDRenderPipeline/RenderPipelineResources/HDRenderPipelineResources.asset

   debugFullScreenShader: {fileID: 4800000, guid: e874aca2df8300a488258738c31f85cf,
     type: 3}
   deferredShader: {fileID: 4800000, guid: 00dd221e34a6ab349a1196b0f2fab693, type: 3}
-  screenSpaceAmbientOcclusionShader: {fileID: 4800000, guid: cf0db7f5267ad944dbf4326b7102c9ca,
-    type: 3}
+  combineLightingPass: {fileID: 4800000, guid: 2e37131331fbdca449b1a2bc47a639ca, type: 3}
-    type: 3}
-  volumetricLightingCS: {fileID: 7200000, guid: 799166e2ee6a4b041bba9e74f6942097,
     type: 3}
   gaussianPyramidCS: {fileID: 7200000, guid: 6dba4103d23a7904fbc49099355aff3e, type: 3}
   depthPyramidCS: {fileID: 7200000, guid: 64a553bb564274041906f78ffba955e4, type: 3}
   deferredComputeShader: {fileID: 7200000, guid: 0b64f79746d2daf4198eaf6eab9af259,
     type: 3}
   deferredDirectionalShadowComputeShader: {fileID: 7200000, guid: fbde6fae193b2a94e9fd97c163c204f4,
+    type: 3}
+  volumetricLightingCS: {fileID: 7200000, guid: 799166e2ee6a4b041bba9e74f6942097,
     type: 3}
   cameraMotionVectors: {fileID: 4800000, guid: 035941b63024d1943af48811c1db20d9, type: 3}
   blitCubemap: {fileID: 4800000, guid: d05913e251bed7a4992c921c62e1b647, type: 3}

ScriptableRenderPipeline/HDRenderPipeline/Sky/HDRISky/Resources/SkyHDRI.shader

     #pragma only_renderers d3d11 ps4 xboxone vulkan metal
 
     #include "ShaderLibrary/Common.hlsl"
+    #include "../../../ShaderVariables.hlsl"
+    #include "ShaderLibrary/VolumeRendering.hlsl"
+#ifdef VOLUMETRIC_LIGHTING_ENABLED
+    SamplerState s_linear_clamp_sampler;
+    TEXTURE3D(_VBufferLighting);
+#endif
 
     float4   _SkyParam; // x exposure, y multiplier, z rotation
     float4x4 _PixelCoordToViewDirWS; // Actually just 3x3, but Unity can only set 4x4
         dir = float3(dot(rotDirX, dir), dir.y, dot(rotDirY, dir));
 
         float3 skyColor = ClampToFloat16Max(SAMPLE_TEXTURECUBE_LOD(_Cubemap, sampler_Cubemap, dir, 0).rgb * exp2(_SkyParam.x) * _SkyParam.y);
+
+    #ifdef VOLUMETRIC_LIGHTING_ENABLED
+        PositionInputs posInput = GetPositionInput(input.positionCS.xy, _ScreenSize.zw);
+        float4 volumetricLighting = GetInScatteredRadianceAndTransmittance(posInput.positionNDC,
+                                                                           _VBufferLighting,
+                                                                           s_linear_clamp_sampler,
+                                                                           _VBufferResolutionAndScale.zw);
+        skyColor *= volumetricLighting.a;
+        skyColor += volumetricLighting.rgb;
+    #endif
+
         return float4(skyColor, 1.0);
     }