Impose a dark gray background for RGB case and a dark gray semi-transparent background for the RGBA case.
Impose a dark gray background for RGB case and a dark gray
semi-transparent background for the RGBA case.

svn path=/trunk/; revision=11202

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