gltfpack: Preserve asset.extras data
gltfpack: Preserve asset.extras data

We now preserve asset.extras that contains information about the author
of the model as well as copyright information, which makes the packed
models compliant with the typical licenses on Sketchfab as long as the
source .gltf was.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer