Backporting robustness patches from Inkscape.
Increase number of max Spiro iterations to 30 since I saw a spline converge at 15.
If did not converge or encountered non-finite values, do not output spline. Only good splines will be output.
Huh. GCC is still stuck in 1989. for loop initial declarations not allowed unless you ask nicely! :-P
Merge pull request #1 from jamadagni/polish

Polishing Spiro