paint method
Implementation
@override
void paint(PCanvas pCanvas, [GraphicContext? graphicContext]) {
  graphicContext = resolveGraphicContext(graphicContext);
  var x = graphicContext.position.x;
  var y = graphicContext.position.y;
  if (backgroundColor != null) {
    pCanvas.fillRect(
        x, y, width, height, PStyle(color: graphicContext.backgroundColor));
  }
  for (var e in elements) {
    e.paint(pCanvas, graphicContext);
  }
}