[gimp] app: avoid deleting empty layer groups when downscaling image

[Ell <ell src gnome org>]

commit b99a2631efd70c0f04bfcc6abc7dfdea31384ba8
Author: Ell <ell_se yahoo com>
Date:   Tue Jan 21 20:01:20 2020 +0200

    app: avoid deleting empty layer groups when downscaling image
    
    When dowscaling an image (or a layer group), empty layer groups
    can be discarded as a result of their new dimensions being too
    small, since we're calculating their new dimensions according to
    their fake 1x1 dimensions.  However, these dimensions are purely an
    implementation detail and shouldn't affect the result, and neither
    do we show a warning for them.
    
    Instead, simply avoid discarding empty layer groups.

 app/core/gimpitem.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/app/core/gimpitem.c b/app/core/gimpitem.c
index 5edaf3804b..56dfa07ada 100644
--- a/app/core/gimpitem.c
+++ b/app/core/gimpitem.c
@@ -31,6 +31,7 @@
 #include "gimp.h"
 #include "gimp-parasites.h"
 #include "gimpchannel.h"
+#include "gimpcontainer.h"
 #include "gimpidtable.h"
 #include "gimpimage.h"
 #include "gimpimage-undo.h"
@@ -1450,6 +1451,7 @@ gimp_item_scale_by_factors_with_origin (GimpItem              *item,
                                         GimpProgress          *progress)
 {
   GimpItemPrivate *private;
+  GimpContainer   *children;
   gint             new_width, new_height;
   gint             new_offset_x, new_offset_y;
 
@@ -1465,6 +1467,12 @@ gimp_item_scale_by_factors_with_origin (GimpItem              *item,
       return FALSE;
     }
 
+  children = gimp_viewable_get_children (GIMP_VIEWABLE (item));
+
+  /* avoid discarding empty layer groups */
+  if (children && gimp_container_is_empty (children))
+    return TRUE;
+
   new_offset_x = SIGNED_ROUND (w_factor * (private->offset_x - origin_x));
   new_offset_y = SIGNED_ROUND (h_factor * (private->offset_y - origin_y));
   new_width    = SIGNED_ROUND (w_factor * (private->offset_x - origin_x +