[gegl] color-reduction: fix off by one in parameters

[Øyvind "pippin" Kolås <ok src gnome org>]

commit e66b1ca59c8fda0f4f567c96be5c5a9fae3384b1
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri Feb 26 16:32:29 2021 +0100

    color-reduction: fix off by one in parameters
    
    The quantization was producing one level more than specified in the
    input, thus 2 levels would yield 3. Fixing issue #278.

 operations/common/dither.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/operations/common/dither.c b/operations/common/dither.c
index c069efd8a..cc66e9d7e 100644
--- a/operations/common/dither.c
+++ b/operations/common/dither.c
@@ -553,10 +553,10 @@ process (GeglOperation       *operation,
   guint       channel_levels [4];
   const Babl *format = gegl_operation_get_format (operation,"output");
 
-  channel_levels [0] = o->red_levels;
-  channel_levels [1] = o->green_levels;
-  channel_levels [2] = o->blue_levels;
-  channel_levels [3] = o->alpha_levels;
+  channel_levels [0] = o->red_levels - 1;
+  channel_levels [1] = o->green_levels - 1;
+  channel_levels [2] = o->blue_levels - 1;
+  channel_levels [3] = o->alpha_levels - 1;
 
   if (o->dither_method != GEGL_DITHER_FLOYD_STEINBERG)
     process_standard (input, output, result, channel_levels,