Add iridescence & anisotropy models to the test suite

[bhouston]

Description

@echadwick-artist has created a series of new models for anisotropy and iridescence testing. We should add them to the test suite. He describes them here:

• Iridescence test model? KhronosGroup/glTF-Sample-Models#392 (comment)
• Anisotropy test model? KhronosGroup/glTF-Sample-Models#393 (comment)

I would like to add all of the models he lists to the model-viewer test suite so that we have proper coverage in these undercovered areas.

Live Demo

None.

Version

None.

Browser Affected

None.

OS

None.

AR

None.

[elalish]

Yes, in fact I bet there are more models too - we haven't been keeping up with sample model updates very well. Not sure when I'll have time, but I'd welcome anyone who wants to drop a PR adding them.

[bhouston]

I've created a pull request here: #4493 for the anisotropy models.

[elalish]

I believe you already added all of these, thanks!