Fix orientation of the inner circular arc extras

[fontanf]

Found while investigating fontanf/packingsolver#595.

Problem

When the line-segment approximation of a shape self-intersects, approximate_by_line_segments falls back to a union of the original shape with "extras": the circular segments between each arc and its chords (or tangents).

compute_circular_arc_extras_inner built these circular segments clockwise (negative area), whatever the arc's own orientation. compute_union expects anticlockwise shapes, which is how compute_circular_arc_extras_outer builds them. For an anticlockwise hole arc, the union then filled the hole entirely instead of replacing its arcs by chords. This is a silently wrong result: on a 100 × 60 rectangle with a circular hole of radius 20, the output had no hole and area 6000.

Fix

Build them anticlockwise: the arc traversed anticlockwise, closed by the chord.

Testing

• New approximate_by_line_segments/4.json case: a 100 × 100 square with a thin half-annulus hole (anticlockwise outer arc of radius 30, clockwise inner arc of radius 25), with segment length 20. At that length, the tangent approximation of the inner arc crosses the chords of the outer arc, so the fallback union is used.
  • On main (with Fix degenerate line segment at the end of an on-circle arc approximation #69), the hole vanishes: the output is the full square, area 10000.
  • With the fix, the hole is kept, as 3 polygonal pieces. The area is 9806.7, above the exact 9568.0, as expected for an approximation that shrinks the hole.
• The rectangle-with-a-circular-hole case from the first version of this PR was dropped: since Fix degenerate line segment at the end of an on-circle arc approximation #69, that input no longer goes through the fallback, so it passes without this fix.
• All 484 tests pass.