Make load gltf public

[jiangheng90]

In some case, there are many gltf like format which I need to parse to make custom loader, so I make load_gltf to a static method public on GltfLoader

[ChristopherBiscardi]

seems reasonable to me. I'm not aware of a way to do multiple-asset-single-binary-blob without manually handling it like this, and glTF handling tends to be complicated comparatively so benefits from being usable like this.

for more context: the older .b3dm file format embeds the glTF (newer format uses glTF extensions). So this would mean an AssetLoader implementation for .b3dm that reads the header and table, then ends up holding a blob of glTF that it also needs to process. So having this be pub makes sense, but also might encourage more people to write custom glTF processors since we don't really have an established way to add glTF extension processing afaik.