Helper function to enumerate all the renderables in the datasource
function _forEachRenderable(callback) {
        if (this._nodesById) {
            for (var key in this._nodesById) {
                callback(this._nodesById[key]);
            }
        }
        else {
            var sequence = this._viewSequence.getHead();
            while (sequence) {
                var renderable = sequence.get();
                if (renderable) {
                    callback(renderable);
                }
                sequence = sequence.getNext();
            }
        }
    }