gltfio: support material extras

[alexey-pelykh]

No description provided.

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[alexey-pelykh]

Change requests have been addressed

[prideout]

I know it's been a loooong time (my apologies) but I actually think we can merge this PR, it's a useful feature and not dangerous.

[alexey-pelykh]

@prideout do you need me to rebase it and/or do anything else?

[prideout]

@prideout do you need me to rebase it and/or do anything else?

Nope, we can merge this after we cut our next release (which will likely be Tuesday).