[gimp] app: avoid NULL output in layer-mode ops

[Ell <ell src gnome org>]

commit 8fcac3298c4947fa1535ae11d6e268e6760d6156
Author: Ell <ell_se yahoo com>
Date:   Fri May 31 12:19:21 2019 -0400

    app: avoid NULL output in layer-mode ops
    
    In GimpOperationLayerMode and GimpOperationReplace, make sure we
    don't return a NULL output buffer, or forward a NULL input buffer,
    but rather create an appropriate empty buffer in this case.  This
    avoids wrong results when the layer-mode op's output is connected
    to the aux input of a subsequent op, as a result of the op behaving
    differently with a NULL aux buffer (in particular, this can happen
    when a drawable filter's output bounding box is smaller than the
    drawable.)

 app/operations/layer-modes/gimpoperationlayermode.c |  7 ++++++-
 app/operations/layer-modes/gimpoperationreplace.c   | 11 +++++++++--
 2 files changed, 15 insertions(+), 3 deletions(-)
---
diff --git a/app/operations/layer-modes/gimpoperationlayermode.c 
b/app/operations/layer-modes/gimpoperationlayermode.c
index 2ec62493a2..5e3352b758 100644
--- a/app/operations/layer-modes/gimpoperationlayermode.c
+++ b/app/operations/layer-modes/gimpoperationlayermode.c
@@ -458,7 +458,12 @@ gimp_operation_layer_mode_parent_process (GeglOperation        *operation,
       else
         {
           /* ... the output is empty. */
-          gegl_operation_context_set_object (context, "output", NULL);
+          gegl_operation_context_take_object (
+            context, "output",
+            G_OBJECT (gegl_buffer_new (result,
+                                       gegl_operation_get_format (operation,
+                                                                  "output"))));
+
           return TRUE;
         }
     }
diff --git a/app/operations/layer-modes/gimpoperationreplace.c 
b/app/operations/layer-modes/gimpoperationreplace.c
index 18111fd402..121801879f 100644
--- a/app/operations/layer-modes/gimpoperationreplace.c
+++ b/app/operations/layer-modes/gimpoperationreplace.c
@@ -96,9 +96,16 @@ gimp_operation_replace_parent_process (GeglOperation        *op,
     {
       GObject *aux;
 
-      aux = gegl_operation_context_get_object (context, "aux");
+      aux = gegl_operation_context_dup_object (context, "aux");
 
-      gegl_operation_context_set_object (context, "output", aux);
+      if (! aux)
+        {
+          aux = G_OBJECT (
+            gegl_buffer_new (result,
+                             gegl_operation_get_format (op, "output")));
+        }
+
+      gegl_operation_context_take_object (context, "output", aux);
 
       return TRUE;
     }