add 3 anisotropy tests as suggested by EricChadwick

[bhouston]

Reference Issue

Fixes the missing anisotropy tests mentioned in this issue: https://github.com/google/model-viewer/issues/4486

This involved:

• Ensuring that glTF-Sample-Assets repository in addition to the pre-existing glTF-Sample-Models (which is the deprecated model repository) is checked out locally and ignored in git. This is where the new models are sourced from.
• Adding 3 anisotropy models to config.json: AnisotropyBarnLamp, AnisotropyRotationTest and AnisotropyStrengthTest.
• Running the screenshot updater and including those results (except for the blank images, there is a bug with three-gpu-pathtracer) as part of this PR as goldens.

[bhouston]

Closing this PR as it has been made obsolete by these two new PRs:

• https://github.com/google/model-viewer/pull/4534
• https://github.com/google/model-viewer/pull/4535