ellipse method
Implementation
void ellipse(double x, double y, double w, double h) {
final rect = new Offset(x - w / 2, y - h / 2) & new Size(w, h);
if (useFill) {
paintCanvas.drawOval(rect, fillPaint);
}
if (useStroke) {
paintCanvas.drawOval(rect, strokePaint);
}
}