fill method
Implementation
void fill( double x, double y, double w, double h ){
_p!.style = ui.PaintingStyle.fill;
_p!.color = ui.Color.fromARGB( _canvasEnv._colorA, _canvasEnv._colorR, _canvasEnv._colorG, _canvasEnv._colorB );
_c!.drawRect( ui.Rect.fromLTWH( x.toDouble(), y.toDouble(), w.toDouble(), h.toDouble() ), _p! );
}