Create: Strokes drew every line with round caps: a square-ended line 10 px wide
had its corners 2.94 px outside the stroke.
- A line's ends are square when its ink reaches as far a third of the width to
either side of its middle as along it (a round end stops short there by a
seventh of the width); told from lines 3 px wide and more, all the ends of a
line together. The stroke then has square caps (`TracedPath::square_caps`).
- The last points of a wide line's skeleton can turn towards a corner of its
end. An end that has left the line the skeleton followed a width back (by a
quarter of the width, 1 px at least) is put back on it; and an end may be
pulled back as well as pushed out to where the ink ends.
Outline on the thin-line inputs: largest distance from the art 2.94 -> 0.88 px
(clean 2.94 -> 0.41; aliased 0.88 -> 0.81), mean 0.055 -> 0.050 px, anchors
14 -> 10.
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