init method
void
init()
Implementation
void init() {
backPaint.style = PaintingStyle.fill;
backPaint.color = Colors.white;
fillPaint.style = PaintingStyle.fill;
fillPaint.color = Colors.white;
strokePaint.style = PaintingStyle.stroke;
strokePaint.color = Colors.black;
strokePaint.strokeCap = StrokeCap.butt;
strokePaint.strokeJoin = StrokeJoin.bevel;
}