[cairo] Bad line joins/cap with ROUND

[Chris Wilson]

Currently when constructing the fan for a ROUND line join/cap we do not
correctly select the termination vertices and sometime choose a vertex
that lies inside the current stroke.

See the attached bad-line-join.

The solution for a water-tight closure of the fan is to include the
termination points in the pen. good-line-join shows the result of
applying the path.

Carl, is this worthy for 1.8? It doesn't address any outstanding issues
but it makes twin look better.
-- 
Chris Wilson
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