If the join indicates the pair of edges are parallel, we may be
considering the final segment of the spline with a different tangent
vector than the slope of the final edge and so lead to false dropping of
an edge. This has the effect that the line segments between 'arc arc arc
arc' (a rounded rectangle) are no longer horizontal or vertical. As path
construction tries to eliminate joins between colinear segments, this
optimisation should not be required anyway.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
