Using a RoundingPen as a filter doesn't work for TrueType quadratic splines (qCurveTo), because the rounding would occur too early, before these get split into atomic quadratic Bezier segments (by the superclass BasePen.qCurveTo method), thus leaving unwanted floating-point coordinates in the SVG output.
So here we add a 'ntos' parameter (by default simpli calls str()) that takes a Callable[[float], str] and can be used to customize the formatting of the numbers in SVG path coordinates.
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