[cairo] Solving the self-intersecting stroke

[Chris Wilson]

Behdad, lets get twin.c into the test suite, it exercises more of the
Bentley-Ottmann tessellator than all of the other tests!

As the keen observer will see, along with some refinement I also
introduced a very nasty bug that was causing trapezoids to be lost.

Carl could you please review the attached patch for correctness. I
believe I've found an issue with _cairo_bo_sweep_line_compare_edges() in
the manner in which it compares the current points along the two edges.
AIUI,
  if (ax.rem < bx.rem)
     return 1;
is a buggy construct as the remainder is a fraction of its divider, so
the correct test would look like:
  if (ax.rem/ady < bx.rem/bdy)
    return 1;
modulo performing the integer arithmetic correctly.
-- 
Chris Wilson
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