Use correct API for DrawPolygon.  Thanks to Joost Kuckartz for spotting this error, and he reports this fix solves an offset issue he had with wxwidgets plots.
Use correct API for DrawPolygon.  Thanks to Joost Kuckartz for
spotting this error, and he reports this fix solves an offset issue he
had with wxwidgets plots.

svn path=/trunk/; revision=12475

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot