HDRenderPipeline: Fix downsample depth to work with reversed depth

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/DepthDownsample.compute

     float2 offset = float2(threadUL) * 2.0f + 1.0f;
     float4 depths = _Source.GatherRed(sampler_PointClamp, offset * _SrcSize.zw, 0.0);
 
+    // Select the nearest sample
+#if UNITY_REVERSED_Z
+    float minDepth = max(min(depths.x, depths.y), max(depths.z, depths.w));
+#else
+#endif
 
     // Write to the final target
     _Result[dispatchThreadId] = minDepth;