A discussion that we've been having is the use of contours in filling,
and for optimizing for convex contours in particular. A devious mind
quickly generates a shape using a convex "contour" whose hull is larger
than its area. This is due to the self-intersection of the "contour"
which if properly excised causes the contour to be classed as concave.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>