arcTo abstract method
forceMoveTo argument is false, adds a straight line
segment and an arc segment.
forceMoveTo argument is true, starts a new sub-path
consisting of an arc segment.
In either case, the arc segment consists of the arc that follows the edge of the oval bounded by the given rectangle, from startAngle radians around the oval up to startAngle + sweepAngle radians around the oval, with zero radians being the point on the right hand side of the oval that crosses the horizontal line that intersects the center of the rectangle and with positive angles going clockwise around the oval.
The line segment added if
forceMoveTo is false starts at the
current point and ends at the start of the arc.
void arcTo(Rect rect, double startAngle, double sweepAngle, bool forceMoveTo);