Fix for bug #1783.  We need to be a lot more careful about when
we clear the image.  In some cases we weren't clearing the image
when we should and in other cases, we were clearing the image when
we shouldn't.  This is exacerbated by the fact that there are three
different ways to paint the three canvases:
-- DGraphView.paint(): used by main network window
-- DGraphView.print(): used by export to graphics routines
-- DGraphView.drawSnapshot(): used by BirdsEyeView

Change-Id: Ib2b7cbef804431c317f12bcf45335792132ab849
Merge branch 'develop' of git+ssh://github.com/cytoscape/cytoscape-impl into develop