add Element::renderTo interface, implement renderElements

#include <atomic>

#include <memory>

#include <string>

#include "utils/return_code.h"

namespace signaltk {

class Layer;

class Element {

public:

  virtual ~Element() = default;

  virtual ReturnCode renderTo(Layer* frame) const = 0;

};

using ElementRef = std::unique_ptr<Element>;

  return buildElementTree(plist, tree);

}

ReturnCode renderElements(

    const ElementTree& tree,

    Layer* frame) {

  for (const auto& e : tree.roots) {

    if (auto rc = e->renderTo(frame); !rc.isSuccess()) {

      return rc;

    }

  }

  return ReturnCode::success();

}

} // namespace signaltk