'Convert to Path' based on the DiaPathRenderer was not producing the
desired effect for some standard objects. Filled objects were not
necessarily filled anymore after conversion. Even simple objects were
producing a group of objects rather than a single path object. Now:
 - start a new path when switching from fill to stroke color use
 - optimize for duplicated path coming from Dia's current split to fill
   and stroke in two steps (with the same path data)
 - be more tolerant regarding path extending so that rounded rectangles
   lead to a single path object
Also included a test/demo file convert-to-path.dia.