[gimp/gimp-2-10] app: do not snap white/black when making 2 color palette

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

commit a0f03c0e3750825cf2188fcf387e96f8e52e0371
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri May 1 17:35:50 2020 +0200

    app: do not snap white/black when making 2 color palette
    
    For such needs there is a dedicated 1bit black/white palette in the
    dialog.
    
    (cherry picked from commit 80664c8e6e3081b71ee80b0d01ee8cd157b9fb27)

 app/core/gimpimage-convert-indexed.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/app/core/gimpimage-convert-indexed.c b/app/core/gimpimage-convert-indexed.c
index 1cc5efd261..ab1457ae14 100644
--- a/app/core/gimpimage-convert-indexed.c
+++ b/app/core/gimpimage-convert-indexed.c
@@ -2812,13 +2812,17 @@ snap_to_black_and_white (QuantizeObj *quantobj)
          }
     }
 
-  if (had_white && white_dist < POW2(32))
+  if (desired > 2 &&
+      had_white   &&
+      white_dist < POW2(128))
   {
      quantobj->cmap[whitest].red   =
      quantobj->cmap[whitest].green =
      quantobj->cmap[whitest].blue  = 255;
   }
-  if (had_black && black_dist < POW2(32))
+  if (desired > 2 &&
+      had_black   &&
+      black_dist < POW2(128))
   {
      quantobj->cmap[blackest].red   =
      quantobj->cmap[blackest].green =