The width between two width points was a straight line, so a variable-width
stroke turned a corner at every width point.
- Each side of a width profile now follows a monotone cubic (Fritsch-Carlson)
through its points: no kink at a point, flat at a point wider or narrower
than both neighbours, never past either point of a span. Points on one line
stay on it and two-point tapers are unchanged; stacked points still step.
- widest() counts each span's wider left and wider right, since the two sides
needn't peak together.
- The outliner samples straight runs between width points closely enough to
follow the curve to the drawing tolerance (at most 64 samples per span and
4,096 per piece); straight spans get none.
- Tests that expected straight-line widths now expect the curve. The stroke
gradient coverage test uses Teardrop instead of Wave until #5964 is fixed.
adambelis