[PlanarReflection] Discard pixels out of oblique frustrum wen samplign environment

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightLoop/LightLoopDef.hlsl

         if (cacheType == ENVCACHETYPE_TEXTURE2D)
         {
             float2 ndc = ComputeNormalizedDeviceCoordinates(_Env2DCapturePositionWS[index] + texCoord, _Env2DCaptureVP[index]);
-            weight = any(ndc < 0) || any(ndc > 1) ? 0 : 1;
-            return SAMPLE_TEXTURE2D_ARRAY_LOD(_Env2DTextures, s_trilinear_clamp_sampler, ndc, index, 0);
+            float4 color = SAMPLE_TEXTURE2D_ARRAY_LOD(_Env2DTextures, s_trilinear_clamp_sampler, ndc, index, 0);
+            // Discard pixels out of oblique projection
+            // We only check RGB because the texture may have BC6H compression
+            weight = any(ndc < 0) || any(ndc > 1) || all(color.rgb >= 1000) ? 0 : 1;
+            return color;
         }
         else if (cacheType == ENVCACHETYPE_CUBEMAP)
         {

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/ReflectionSystemInternal.cs

-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using UnityEngine.Rendering;
 
 namespace UnityEngine.Experimental.Rendering.HDPipeline.Internal
+        // -FLT_MAX in hlsl
+        static readonly Color k_DiscardedColor = new Color(1000, 1000, 1000, 1000);
+
         static Camera s_RenderCamera = null;
         static HDAdditionalCameraData s_RenderCameraData;
 
             camera.fieldOfView = fov;
             camera.aspect = aspect;
             camera.clearFlags = clearFlags;
-            camera.backgroundColor = backgroundColor;
+            // TODO: Find a cleaner way to discard pixel out of frustrum
+            camera.backgroundColor = k_DiscardedColor;
             camera.projectionMatrix = projection;
             camera.worldToCameraMatrix = worldToCamera;