[glTF] support image URIs that use JSON escaping

[footballhead]

If image.uri represents a path, it may encode certain characters with JSON escaping. From glTF2.0 spec, section 2.8:

Paths with non-ASCII characters MAY be written as-is, with JSON string escaping, or with percent-encoding; all these options are valid

I'm not aware of any glTF-Sample-Assets that use this.

cgltf provides the cgltf_decode_string function which may help here to remove the JSON escaping.