Make the "radius" property of CircleGeometricTransform relative.
This is more coherent with the way [x,y]-center properties are handled
and allow to set a radius without knowing the video size.
Radius is defined with respect to the circle circumscribed about the
video rectangle so that a point in the center has radius 0.0 and one in
a vertex has radius 1.0.
Note that this is not a regression from the previous absolute way of
defining the radius as a user who knows the video size can easily
calculate the relative radius and set that.
https://bugzilla.gnome.org/show_bug.cgi?id=625959
