use fastopt with metal, avoid MTLCompilerService sandbox process crash

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

     float3 totalRadiance = 0;
     float  opticalDepth  = 0;
 
-    uint sliceCountHack = max(VBUFFER_SLICE_COUNT, (uint)_VBufferDepthEncodingParams.w); // Prevent unrolling...
-
 #ifdef LIGHTLOOP_TILE_PASS
     // Our voxel is not necessarily completely inside a single light cluster.
     // Note that Z-binning can solve this problem, as we can iterate over all Z-bins
     clusterDepths[0]  = GetLightClusterMinLinearDepth(posInput.tileCoord, clusterIndices[0]);
 #endif // LIGHTLOOP_TILE_PASS
 
+#if defined(SHADER_API_METAL)
+    [fastopt]
+    for (uint slice = 0; slice < VBUFFER_SLICE_COUNT; slice++)
+#else
+    uint sliceCountHack = max(VBUFFER_SLICE_COUNT, (uint)_VBufferDepthEncodingParams.w); // Prevent unrolling...
+
+#endif
+#if defined(SHADER_API_METAL)
+        float z1 = DecodeLogarithmicDepth(e1, _VBufferDepthDecodingParams);
+#else
+#endif
         float t1 = ray.strataDirInvViewZ * z1; // Convert view space Z to distance along the stratified ray
         float dt = t1 - t0;