*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
