For all versions of GD:
For all versions of GD:

1) Remodelled the colour allocation

2) Added a command line "optimise" option (via -DRVOPT) for PNG images. If
   successful, the optimise command will create 4 bit (16 colour) PNGs
   instead of 8 bit (256 colour) ones.

3) Stuff that used to be evoked through compile time #defines and the
   command line option "-hack" are now done via -drvopt.

3a) "-drvopt def_black15" sets index 15, usually white, to black if index
    0, the background colour and usually black, has been set to white from
    the command line option -bg.

3b) "-drvopt swp_red15" swaps index 15, usually white, with index 1, which
    is usually red.

New if you are using V2.+ of GD

4) Added support 24 bit colour

4a) 24 bit palettes now supported if you have GD 2.+  24 bit support is,
    by default, set to "auto", so the *driver* decides when to use 24 bit or
    8 bit modes for PNG files.

4b) "-drvopt 8bit" forces the driver to use 8bit PNGs even if conventional
    wisdom would suggest 24 bit ones would be better.

4c) "-drvopt 24bit" forces the driver into 24 bit mode regardless of how
    many colours are used.

svn path=/trunk/; revision=3869

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