This was requested in https://github.com/googlei18n/cu2qu/issues/26
Plus:
- do not modify input glyphs unless they contain one cubic curve;
- make public functions return True/False to signal that the input
was modified or not (eg. no curves, or all quadratic)
For some reason, I was subtracting 1 from the spline lengths in the
test report. Not sure why that is, so I've assumed it was wrong (and
now we subtract 2 to get the length in number of segments).
