completeLoad = async () => {
    const id = billboardTexture._id;
    const atlas = billboardTexture._billboardCollection.textureAtlas;
    await atlas._indexPromiseById.get(id);

    // Any errors should have already been handled
    if (target._id !== id) {
      // Another load was initiated and resolved resolved before this one. This operation is cancelled.
      return;
    }

    if (billboardTexture._hasSubregion) {
      // Subregions must wait an additional frame to be ready
      await Promise.resolve();
    }

    target._id = id;
    target._loadState = billboardTexture._loadState;
    target._loadError = billboardTexture._loadError;
    target._index = billboardTexture._index;
    target._width = billboardTexture._width;
    target._height = billboardTexture._height;
    target.dirty = true;
  }
Clones an existing billboard texture, inlcuding any in-flight tracking, into the target billboard texture.
@param {BillboardTexture} billboardTexture
@param {BillboardTexture} target
@returns {BillboardTexture} target