Roughen's jitter can put a point behind its predecessor, so the path
turns back. The Catmull-Rom tangent next - prev then shrinks to almost
nothing and the anchor becomes a cusp, which PDF viewers (MuPDF,
poppler, Ghostscript) draw with a miter spike; the preview doesn't.
catmull_rom now keeps the tangent at least half as long as the longer
chord, and where it is shorter (turns of more than about 150 degrees)
points it along the bisector of the two chords: the turn becomes a round
hairpin. Gentler turns keep the plain Catmull-Rom tangent, so Zig Zag,
Scribble and the render goldens are unchanged.
Fixes#1643