must return all 4 components for RGBm

must return all 4 components for RGBm

Assets/ScriptableRenderPipeline/common/CubeToSpherical.shader

     uint2 pixCoord = ((uint2) i.vertex.xy);
 
     half3 dir = SphericalTexCoordinateToDirection(i.texcoord.xy);
-    half3 res = UNITY_SAMPLE_TEXCUBE_LOD(_srcCubeTexture, dir, (float) _cubeMipLvl).xyz;
-
-    return half4(res,1.0);
+    
+    return (half4) UNITY_SAMPLE_TEXCUBE_LOD(_srcCubeTexture, dir, (float) _cubeMipLvl);
 }
 
 ENDCG