circle method
Implementation
void circle( double cx, double cy, double r ){
_p!.style = ui.PaintingStyle.stroke;
_p!.strokeWidth = _canvasEnv._strokeWidth;
_p!.color = ui.Color.fromARGB( _canvasEnv._colorA, _canvasEnv._colorR, _canvasEnv._colorG, _canvasEnv._colorB );
_c!.drawCircle( ui.Offset( cx, cy ), r, _p! );
}