byte_offset and extras of gltf-json::accessor::Accessor should be Option type

[J-F-Liu]

So that the fields don't appear in serialized json.

[aloucks]

I think it would be better to use the serde(skip_serializing) attribute instead.

[J-F-Liu]

yes, like #[serde(default, skip_serializing_if = "is_primitive_mode_default")]