Each band segment was a rectangle ending square at its vertex, so the
outer corner of every joint showed a notch. Every segment end now carries
the tangent of half its turn, and the shader slides that end's corners
onto the joint's bisector so neighbours meet in a miter. Turns sharper
than the limit keep butt ends. Constant-width runs inside a polyline split
around its arcs no longer count as tapered, which had kept them unmitred.
Fixes#1565
Co-Authored-By: Claude Opus 5.5 <[email protected]>