Add UnityGLTFShaderVariants for issue: https://github.com/KhronosGrou…

[Vytek]

…p/UnityGLTF/issues/206

See: https://github.com/KhronosGroup/UnityGLTF/issues/206

This include: UnityGLTFShaderVariant in UnityGLTF/Assets/Resources/ but also in GraphicsSettings.

Please check all ok!

[Zichuan-Wang]

Thanks for the PR!

[blgrossMS]

Sorry for the delay on responding to this. @JohnCopicMS had some opinions about how to deal with this issue in the repo