*plug-ins/ edge.c
                added a check so it woudlnt segfault when
        passed a non-existend drawable

        threshold_alpha  added a non-interactive mode

        *app/brightness_contrast.c
         app/color_balance.c
         app/colormaps.c
         app/curves.c
         app/hue_saturation.c
         app/posterize.c
         app/threshold.c

                -added a check so it wouldnt except
         a indexed drawable. This was previosly possible via the pdb.

-adrian