Fixed positioning support for Circle fixture

The x and y properties of the Circle fixture were being ignored
since change 272a3207fc8e71c67c3a08b3727db057bdecfab6. This change
makes it so that they are being taken into account again, while still
shifting the b2CircleShape to center in the Circle fixture item, at
least when it has its default size.

Instead of setting the width and height explicitly, the implicit
width is now set based on the radius.