Path._curve() method draws a cubic Bézier curve
from the current position via two control points to a given endpoint.
However, the start control point is identical to the current position,
so you do not need to provide it.
Path path._curve(Point cp2, Point to)
This method is chainable as it returns the
The main purpose of this method is to save your some typing, as the two following calls yield the same result:
path.curve(point1, point1, point2) path._curve(point1, point2)