Color space transform in the UI composition is incorrect on DX12 HDR (r.HDR.UI.CompositeMode=1)

Color space conversion in the UI composition is incorrect on DX12 HDR.

In the main() function In CompositeUIPixelShader.usf, you can see the following color space transform.

	// sRGB -> RRT -> ODT
	UIColor.xyz = ColorLookupTable(UIColor.xyz);

But it seems that the input of ColorLookupTable() must be linear color space. So it makes UI color incorrect on HDR.
[Link Removed]
[Link Removed]

Adding sRGB->Linear transform before the function fix this problem.

	// sRGB -> Linear -> RRT -> ODT
	UIColor.xyz = sRGBToLinear(UIColor.xyz);
	UIColor.xyz = ColorLookupTable(UIColor.xyz);

[Link Removed]

1. Open the attached repro project
2. Play Standalone Game, you can see SDR result.
3. set r.HDR.EnableHDROutput to 1 via console command.
4. Play Standalone Game, you can see HDR result. and you can find the result slight brighter than SDR

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