Add support for glTF normalTextureInfo.scale

[jjhembd]

Description

The glTF specification describes a scale that can be applied to the values read from a normal texture. This scalar applies to the .x and .y components only.

This PR adds a .scale property to ModelComponents.TextureReader, and adds code to process the scale in GltfLoaderUtil and MaterialPipelineStage. In the shader, the scalar is applied (if present) in MaterialStageFS.

Here is the Clearcoat Car Paint test model in CesiumJS 1.118:

With this PR:

Comparing to the glTF Sample Viewer

Issue number and link

Resolves #6948.

Testing plan

Load the glTF PBR Extensions Sandcastle and compare the Clearcoat Car Paint model to the rendering in the glTF Sample Viewer

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Looks good to me @jjhembd! Should we add or update any unit tests?

[jjhembd]

Thanks @ggetz, I added/updated some specs. This is ready for another look.

[ggetz]

Awesome, thanks @jjhembd!