[gimp] Revert "app: do not forbid merging down floating layers, instead anchor them."

[Michael Natterer <mitch src gnome org>]

commit d91487e6af004de6496f2c0aefb549cff902d15b
Author: Michael Natterer <mitch gimp org>
Date:   Sun Dec 31 13:56:00 2017 +0100

    Revert "app: do not forbid merging down floating layers, instead anchor them."
    
    This reverts commit 40bc5307dcac2ad47f78a4caed0d564d5c21e836.
    
    It's not *exactly* the same. The floating selection can belong to a
    channel or layer mask. Also, this belongs into the GUI layer, not the
    core.

 app/core/gimpimage-merge.c |   26 ++++++++++----------------
 1 files changed, 10 insertions(+), 16 deletions(-)
---
diff --git a/app/core/gimpimage-merge.c b/app/core/gimpimage-merge.c
index a26724b..54c72e7 100644
--- a/app/core/gimpimage-merge.c
+++ b/app/core/gimpimage-merge.c
@@ -239,6 +239,13 @@ gimp_image_merge_down (GimpImage      *image,
   g_return_val_if_fail (GIMP_IS_CONTEXT (context), NULL);
   g_return_val_if_fail (error == NULL || *error == NULL, NULL);
 
+  if (gimp_layer_is_floating_sel (current_layer))
+    {
+      g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
+                           _("Cannot merge down a floating selection."));
+      return NULL;
+    }
+
   for (list = gimp_item_get_container_iter (GIMP_ITEM (current_layer));
        list;
        list = g_list_next (list))
@@ -291,22 +298,9 @@ gimp_image_merge_down (GimpImage      *image,
                                GIMP_UNDO_GROUP_IMAGE_LAYERS_MERGE,
                                C_("undo-type", "Merge Down"));
 
-  if (gimp_layer_is_floating_sel (current_layer))
-    {
-      /* Merging down a floating selection is basically equivalent to
-       * anchoring it.
-       */
-      floating_sel_anchor (current_layer);
-      /* layer is already set to the right layer below the floating
-       * selection, on which we anchored. This will be the return value.
-       */
-    }
-  else
-    {
-      layer = gimp_image_merge_layers (image,
-                                       gimp_item_get_container (GIMP_ITEM (current_layer)),
-                                       merge_list, context, merge_type);
-    }
+  layer = gimp_image_merge_layers (image,
+                                   gimp_item_get_container (GIMP_ITEM (current_layer)),
+                                   merge_list, context, merge_type);
   g_slist_free (merge_list);
 
   gimp_image_undo_group_end (image);