Function fit_to_bezpath_opt_inner

fn fit_to_bezpath_opt_inner(
    source: &impl ParamCurveFit,
    accuracy: f64,
    range: Range<f64>,
    path: &mut BezPath,
) -> Option<f64>

Fit a range without cusps.

On Ok return, range has been added to the path. On Err return, report a cusp (and don’t mutate path).