Small changes to make the new wxWidgets driver run also for wxGTK 2.6.4, where wxGraphicsContext is not available.
Small changes to make the new wxWidgets driver run also for wxGTK 2.6.4, where wxGraphicsContext is not available.

svn path=/trunk/; revision=8437

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