Fix #127: circular fisheye scaling factor.
Fix #127: circular fisheye scaling factor.

This fixes the scaling value `z` when the focus point is exactly on an
object’s position.

Also fixes #126.

HunjaeJung/d3-plugins,yinchanted/d3-plugins,d3/d3-plugins,sidhart/d3-plugins,larskotthoff/d3-plugins,gronke/d3-plugins,larskotthoff/d3-plugins,d3/d3-plugins,fmacias64/d3-plugins,kaktus40/d3-plugins,johnochs/d3-plugins,fmacias64/d3-plugins,grepman/d3-plugins,ddrugg/d3-plugins,johnochs/d3-plugins,sidhart/d3-plugins,grepman/d3-plugins,ddrugg/d3-plugins,deldersveld/d3-plugins,yinchanted/d3-plugins,kaktus40/d3-plugins,gronke/d3-plugins,gweinert/d3-plugins,HunjaeJung/d3-plugins,gweinert/d3-plugins,deldersveld/d3-plugins