Change mem driver so that it exits if it is asked to plot with alpha values.
Change mem driver so that it exits if it is asked to plot with alpha values.

svn path=/trunk/; revision=11195

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