code: boolean onSplitWork(int bytesMoved) {
          if (_splitTargetRemaining <= 0) {
              _splitTargetRemaining = _splitTargetSize - bytesMoved;
              return true;
          } else {
              _splitTargetRemaining -= bytesMoved;
              return false;
          }
      }
docs: Tracks the # of bytes that have been moved.  Returns true if the splitter should create a new destination segment.