glTF JSON, buffers, and images may come from a range of external sources, so
customization is an important design goal of the import module. The Source
trait is provided to facilitate customization of the data loading process.
For convenience, the library contains one implementation of the Source trait,
namely FromPath, which allows loading from file system and also from embedded
base64 encoded data. This implementation may be used as reference for other
schemes such as http.