function usesExtension(gltf, extension) {
  return (
    defined(gltf.extensionsUsed) && gltf.extensionsUsed.indexOf(extension) >= 0
  );
}
Checks whether the glTF uses the given extension.

@param {object} gltf A javascript object containing a glTF asset.
@param {string} extension The name of the extension.
@returns {boolean} Whether the glTF uses the given extension.

@private