Add dev_clear, so drivers can signal if PLESC_CLEAR is supported.
Add dev_clear, so drivers can signal if PLESC_CLEAR is supported.

svn path=/trunk/; revision=4036

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