Add copyright to GLTFState

[aaronfranke]

This PR adds a copyright property to GLTFState which can be used to set the GLTF copyright string in the asset header (asset.copyright). https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/schema/asset.schema.json

I also organized the asset header code, adding a new method _parse_asset_header and renaming _serialize_version to _serialize_asset_header.

For example, during export this can be done:

gltf_state.copyright = "2014 Godette"

Which results in the GLTF containing this text:

"asset": {
	"copyright": "2014 Godette",
	"version": "2.0"
},

And it can be read back at import time:

print("Copyright " + gltf_state.copyright) # Copyright 2014 Godette

[aaronfranke]

@fire I am working on a system for specifying export settings like copyright, this PR is a part of that work actually.

[fire]

@aaronfranke I want to mention that the xmp extension is what khronos and adobe, and all the stores recommend for this usage.

Exporting field didn’t fit their needs.

[akien-mga]

Thanks!