function addExtensionsUsed(gltf, extension) {
  let extensionsUsed = gltf.extensionsUsed;
  if (!defined(extensionsUsed)) {
    extensionsUsed = [];
    gltf.extensionsUsed = extensionsUsed;
  }
  addToArray(extensionsUsed, extension, true);
}
Adds an extension to gltf.extensionsUsed if it does not already exist.
Initializes extensionsUsed if it is not defined.

@param {object} gltf A javascript object containing a glTF asset.
@param {string} extension The extension to add.

@private