PyDia: fix wrong cast to Diagram* were DiagramData* was intended
pstricks: optimize draw_polygon/draw_beziergon

Also finally adapted to the even-odd-fill Dia is using.
Looking good with convert-to-path although some general
clipping problem still exists.
pstricks: fix PstricksRenderer::draw_image() to not skew the image
pstricks: fixed linestyles for dash-dot and dash-dot-dot

Formerly these were breaking the rendering altogether, now only the
text rendering has still issues.
PyDia: fix wrong cast to Diagram* were DiagramData* (now really)

PyDiaDiagramData_GetAttr() can deliver attributes from Diagram and
DiagramData. In the stand alone or command line case these are not
necessarily the same. E.g. file conversion is usually only done
with DigramData.
DiaRenderer: replace fill_polygon with extended draw_polygon

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

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

Not yet optimized are: PGF, VDX, XFig

Also not optimized, but more simple are: GDK, Cairo, DXF, HPGL, LibArt, PS,
  Pstricks,

Optimized to fill and stroke in one step are: Import, Path, SVG, Transform,
  DRS, Metapost, diadissect.py, diastddia.py, diasvg.py, PyDia, Shape, WMF,
  WPG

Fully optimized shape rendering for all objects except with active pattern.