The rows' spans were all chained into one polyline, so where a row had
more than one span the line ran across the hole between them, outside
the shape. A span now continues the previous row's chain whose
connecting line stays inside the shape (the nearest one); otherwise it
starts a chain of its own. In a convex shape the connector always stays
inside, so the result there is unchanged (render goldens pass).
Fixes#1648
Gert Pellin
85335e9e14