gltfpack: Support EXT_structural_metadata, EXT_mesh_features

[rhonke]

Hi there,

please support EXT_structural_metadata and EXT_mesh_features which seem not to survive compression at the moment.

Thank you!

[zeux]

These extensions have not been merged; they are also multi-vendor, so support is not guaranteed. I have briefly looked at them in the past and it might make sense to support partially; however, for that to happen the minimum bar is:

• Their specification should be merged to glTF repository
• They should be supported by https://github.com/jkuhlmann/cgltf

Once that is done, gltfpack support would be possible to evaluate further.