Alpha blending is done twice when using RenderOpacity of RetainerBox's parent slot

Licensee reported that alpha blending is done twice in the code below, and the fix below seems to fix it.

.\Engine\Source\Runtime\UMG\Private\Slate\SRetainerWidget.cpp
From:

const FLinearColor ComputedColorAndOpacity(Context.WidgetStyle.GetColorAndOpacityTint() * GetColorAndOpacity() * SurfaceBrush.GetTint(Context.WidgetStyle));

To:

FLinearColor ContextWidgetStyleColor = Context.WidgetStyle.GetColorAndOpacityTint();
ContextWidgetStyleColor.A = 1.0f;
const FLinearColor ComputedColorAndOpacity(ContextWidgetStyleColor * GetColorAndOpacity() * SurfaceBrush.GetTint(Context.WidgetStyle));

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1. Open the attached project
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   This project contains the following setup:
   *Set RenderOpacity of CanvasPanel to 0.5.
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   *The materials used for RetainerBox are as follows.
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2. Play. A RetainerBox that contains Alpha will be faded.
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