Add simple examples to ease testing

Most examples are from [SVGround](svground.fr).
SVG Ellipse: generalization of SVG Circle

An ellipse is a circle with two radius parameters rx and ry.
The Circle class inherits from the Ellipse class, as a circle is a
special case of ellipse (rx = ry = r).

As a side effect, circle scaling with xscale != yscale is now supported.