More careful splitting of glTF materials

[emackey]

This addresses a bug found in #6412 where Polly, the robot dog, lost all her lighting, even emissive lights.

Some materials were used with and without tangents, or with and without morph targets, and needed to be split apart. Also, the hasTangents and hasMorphTargets booleans needed to be isolated to their respective materials (after splitting).

[cesium-concierge]

@emackey, thanks for the pull request! Maintainers, we have a signed CLA from @emackey, so you can review this at any time.

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[lilleyse]

@emackey looks good, just update CHANGES.md.

[emackey]

Actually, going to add one more tweak here, hold on...

[emackey]

Updated. Now fixes #6061.

This is ready.

[lilleyse]

Nice - Polly and Alien work great.