Fix Issue #0002522: Rendering a clustered digraph twice produces different result (filled clusters).
Fix Issue #0002522: Rendering a clustered digraph twice produces different result (filled clusters).

pixelglow/graphviz,BMJHayward/graphviz,jho1965us/graphviz,BMJHayward/graphviz,kbrock/graphviz,ellson/graphviz,pixelglow/graphviz,ellson/graphviz,pixelglow/graphviz,jho1965us/graphviz,pixelglow/graphviz,MjAbuz/graphviz,jho1965us/graphviz,jho1965us/graphviz,kbrock/graphviz,pixelglow/graphviz,ellson/graphviz,MjAbuz/graphviz,jho1965us/graphviz,pixelglow/graphviz,BMJHayward/graphviz,BMJHayward/graphviz,jho1965us/graphviz,kbrock/graphviz,kbrock/graphviz,kbrock/graphviz,jho1965us/graphviz,jho1965us/graphviz,MjAbuz/graphviz,jho1965us/graphviz,jho1965us/graphviz,kbrock/graphviz,kbrock/graphviz,MjAbuz/graphviz,pixelglow/graphviz,ellson/graphviz,BMJHayward/graphviz,ellson/graphviz,MjAbuz/graphviz,pixelglow/graphviz,pixelglow/graphviz,jho1965us/graphviz,kbrock/graphviz,ellson/graphviz,MjAbuz/graphviz,MjAbuz/graphviz,MjAbuz/graphviz,pixelglow/graphviz,BMJHayward/graphviz,BMJHayward/graphviz,ellson/graphviz,BMJHayward/graphviz,ellson/graphviz,BMJHayward/graphviz,kbrock/graphviz,BMJHayward/graphviz,ellson/graphviz,MjAbuz/graphviz,BMJHayward/graphviz,pixelglow/graphviz,MjAbuz/graphviz,kbrock/graphviz,kbrock/graphviz,ellson/graphviz,ellson/graphviz,MjAbuz/graphviz