Core: Explicitly merge down NDE filters on flatten

Resolves #10769.

Flattening an image or merging down layers caused
us to lose NDE filters on undo, as we flattened with
gimp_gegl_create_flatten_node () which didn't create
an Undo/Redo object.
To fix this, we merge the filters down beforehand with
our API so that they will be restored when undoing.

app/core/gimpimage-merge.c

@@ -36,6 +36,7 @@
 
 #include "gimp.h"
 #include "gimpcontext.h"
+#include "gimpdrawable-filters.h"
 #include "gimperror.h"
 #include "gimpgrouplayer.h"
 #include "gimpimage.h"
@@ -601,6 +602,10 @@ gimp_image_merge_layers (GimpImage     *image,
 
       layer = layers->data;
 
+      /* Merge down filter effects so they can be restored on undo */
+      gimp_drawable_merge_filters (GIMP_DRAWABLE (layer));
+      gimp_drawable_clear_filters (GIMP_DRAWABLE (layer));
+
       gimp_item_get_offset (GIMP_ITEM (layer), &off_x, &off_y);
 
       switch (merge_type)