code: public QuadBatch pushBatch (QuadBatch newBatch) {
      if (newBatch == null) return null;
      QuadBatch oldBatch = batch;
      batch.end();
      batch = beginBatch(newBatch);
      return oldBatch;
    }
docs: Configures this surface to use {@code batch}, if non-null. NOOPs otherwise.
  @return a batch which should be passed to {@link #popBatch} when rendering is done with this
  batch.