[gimp/gimp-2-10] app: avoid deleting empty layer groups when downscaling image

[Ell <ell src gnome org>]

commit 4f9c3f2a6a005f7fc89384e58de057c20a6e7e51
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.
    
    (cherry picked from commit b99a2631efd70c0f04bfcc6abc7dfdea31384ba8)

 app/core/gimpitem.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/app/core/gimpitem.c b/app/core/gimpitem.c
index 6960d0b461..c0f732c078 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"
@@ -1456,6 +1457,7 @@ 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;
 
@@ -1471,6 +1473,12 @@ 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 +