`unite_all` and `try_unite_all` swept each input under its own fill rule
and then swept all the resulting contours again under NonZero. With one
input whose sweep split and dropped none of its segments, the second
sweep ran over the same simple contours and doubled the time. Such an
input is now refit from the first sweep's contours. Inputs that cross
themselves and unions of several inputs are still swept again, and the
input paths are converted once instead of twice.
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