Need strict definition of black border color in UsdPreviewSurface Texture Reader

[syoyo]

Description of Issue

https://openusd.org/release/spec_usdpreviewsurface.html#version-2-5-current-head

The spec says

Wrap mode when reading this texture.Possible Values:

        black : Reader returns black outside unit square

But not sure what actual black color should be.

Possible value is either transparent black (0, 0, 0, 0) or opaque black (0, 0, 0, 1)

We need strict definition of black border color.
(NOTE: Currently TinyUSDZ Tydra uses transparent black)

I'm not sure how Storm interprets black border color, but it seems Storm uses TransparentBlack as a default border color: https://github.com/PixarAnimationStudios/OpenUSD/blob/0b18ad3f840c24eb25e16b795a5b0821cf05126e/pxr/imaging/hgi/sampler.h#L73C18-L73C18

cc @dgovil

Steps to Reproduce

N/A

System Information (OS, Hardware)

N/A

Package Versions

23.08

Build Flags

N/A

[sunyab]

Filed as internal issue #USD-8890