message: Fix refinement logic.
  Empty tiles are renderable, but external tilesets are not.
diff: @@ -124,10 +124,16 @@ namespace Cesium3DTiles {
  }
  bool Tile::isRenderable() const {
  + // A tile whose content is an external tileset has no renderable content. If
  + // we select such a tile for rendering, we'll end up rendering nothing even though
  + // the tile's parent and its children may both have content. End result: when the
  + // tile's parent refines, we get a hole in the content until the children load.
  +
  + // So, we explicitly treat external tilesets as non-renderable.
  +
  return
  this->getState() >= LoadState::ContentLoaded &&
  - this->_pContent &&
  - this->_pContent->getType() != ExternalTilesetContent::TYPE;
  + (!this->_pContent || this->_pContent->getType() != ExternalTilesetContent::TYPE);
  }
  void Tile::loadContent() {
  