rx and ry of rect
For two blue rectangles the attributes rx and ry are animated with additive sum.
The animation is compared for the top left corner of each
rectangle with an animated ellipse with the same rx and ry.
If the top left quarters of the stroked ellipses do not fit exactly
to the top left rounded corner of the related rectangle, an error
is occured.
In the specification it is not mentioned, which value is the underlying value,
if there is no lower priority animation or a static value. Here it is assumed to
be 0. The other explicitely given r value cannot be used as underlying value,
when the correction is only done on the presentation value as usual for
SMIL animations.