Add GLTF support for second UV channel

[RunitaiLinden]

Test plan

1. Download each of the assets from the glTF folder of the Test model(s).
2. Create a box in Second Life.
3. While the box is selected in edit mode, click Develop menu > GLTF > Open....
4. Choose the .gltf file of the Test model.
5. Verify the model appears in-world and matches the Screenshot on Test model page.

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Per GLTF spec:

Client implementations SHOULD support at least two texture coordinate sets, one vertex color, and one joints/weights set.

We currently only support one texture coordinate set. Add support for a second UV channel.

Test models: https://github.com/KhronosGroup/glTF-Sample-Models/tree/main/2.0/MultiUVTest https://github.com/KhronosGroup/glTF-Sample-Models/tree/main/2.0/TextureTransformMultiTest

[brad-linden]

should this be marked integrated?

[vladyslavasproductengine]

Fail QA.

Verified on the Second Life Release 7.1.9.9620320242 (64bit) on Win10/OSX in scope https://github.com/secondlife/iqa/issues/266.

Models do not appear in-world and matches the Screenshot on Test model page see screenshots.

screenshot1: screenshot2: