Don't use 0 as the default value for fields

[David Dubois]

fastgltf seems to use 0 as the default value for various fields - possibly because you're not setting an explicit default value. This has caused a bunch of issues we I'm trying to create glTF files. I'd like to request that fastgltf only use 0 as a default value when it's what the glTF spec says as the default, and use a different value when appropriate