clear method
Implementation
void clear( [int? x, int? y, int? w, int? h] ){
_p!.style = ui.PaintingStyle.fill;
_p!.color = ui.Color.fromARGB( _canvasEnv._colorA, _canvasEnv._colorR, _canvasEnv._colorG, _canvasEnv._colorB );
if( (x == null) && (y == null) && (w == null) && (h == null) ){
_c!.drawRect( ui.Rect.fromLTWH( 0, 0, _width.toDouble(), _height.toDouble() ), _p! );
} else if( (w == null) && (h == null) ){
_c!.drawRect( ui.Rect.fromLTWH( x!.toDouble(), y!.toDouble(), 1, 1 ), _p! );
} else {
_c!.drawRect( ui.Rect.fromLTWH( x!.toDouble(), y!.toDouble(), w!.toDouble(), h!.toDouble() ), _p! );
}
}