[path] "Invert Path" to change the path direction
metapost: optimize draw_polygon/draw_beziergon, dash length tweaking

Polygon and Beziergon are now properly closed with draw_*gon() call. Also
the path created is used for fill and stroke.

Dash length was nearly producing completely stroked lines with
render-test.dia now it looks a bit better for most lines.

Hint for testing:

Sorry, I can't find the 'metafun' preload file; will try 'plain'.
I can't find the 'plain' preload file!

-> sudo apt-get install texlive-metapost

It still does not complile unchanged because of unescaped (),
so that text has to be deleted.
pgf: fix some comment