Add pngcairo driver to plplot tests - useful for debugging current  problems with alpha support.
Add pngcairo driver to plplot tests - useful for debugging current 
problems with alpha support.


svn path=/trunk/; revision=8155

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