radius property
The radius of this RadialSteps as a fraction of
the shortest side of the paint box.
For example, if a RadialSteps is painted on a box that is
100.0 pixels wide and 200.0 pixels tall, then a radius of 1.0
will place the 1.0 stop at 100.0 pixels from the center.
Copied from RadialGradient.
Implementation
final double radius;