What started with draw_beziergon() is continued with draw_polygon(): merge
two calls into one to get rid of superfluous drawing commands and arbitrary
object splits.

Given that fill_polygon was a required method now draw_polygon is. So it was
implemented more often than fill_bezier

To reduce the risk of breaking file formats not yet tested the following
pattern is introduced:
 - rename draw_polygon to stroke_polygon
 - keep fill_polygon as local function
 - make new draw_polygon call one or both for fill and stroke parameter
As a result the output of these exporters wont change at all.

Not optimized are: MetaPost, LibArt, XFig, VDX, diastddia.py
Also not optimized, but more simple are: PGF, PS, PSTricks and Skia

Optimized to fill and stroke in one step are: WPG, PyDia, WMF, CGM, DRS,
  cairo, SVG, Shape, GDK, HPGL(kind of;)) and DXF(as limited as before)

Fully optimized shape rendering for: AADL, Custom, KAOS, Standard,
  Flowchart, Network, ER, Chronogram, UML - i.e. all programmed objects