[gimp] app: make "Alpha to Selection" multi-layer aware.

[Jehan <jehanp src gnome org>]

commit 645d80123acf8eb83f44795a0ff0bb8746b28cc8
Author: Jehan <jehan girinstud io>
Date:   Mon May 18 12:22:08 2020 +0200

    app: make "Alpha to Selection" multi-layer aware.
    
    Allows to select several layers and have their alpha channel to replace
    the selection, or substract from it, etc.

 app/actions/layers-actions.c  |  8 ++++----
 app/actions/layers-commands.c | 17 +++++++++++++----
 2 files changed, 17 insertions(+), 8 deletions(-)
---
diff --git a/app/actions/layers-actions.c b/app/actions/layers-actions.c
index 57f3bbbfa5..f1ef10a145 100644
--- a/app/actions/layers-actions.c
+++ b/app/actions/layers-actions.c
@@ -1094,10 +1094,10 @@ layers_actions_update (GimpActionGroup *group,
   SET_SENSITIVE ("layers-mask-selection-subtract",  layer && !fs && !ac && mask);
   SET_SENSITIVE ("layers-mask-selection-intersect", layer && !fs && !ac && mask);
 
-  SET_SENSITIVE ("layers-alpha-selection-replace",   layer && !fs && !ac);
-  SET_SENSITIVE ("layers-alpha-selection-add",       layer && !fs && !ac);
-  SET_SENSITIVE ("layers-alpha-selection-subtract",  layer && !fs && !ac);
-  SET_SENSITIVE ("layers-alpha-selection-intersect", layer && !fs && !ac);
+  SET_SENSITIVE ("layers-alpha-selection-replace",   n_layers > 0 && !fs && !ac);
+  SET_SENSITIVE ("layers-alpha-selection-add",       n_layers > 0 && !fs && !ac);
+  SET_SENSITIVE ("layers-alpha-selection-subtract",  n_layers > 0 && !fs && !ac);
+  SET_SENSITIVE ("layers-alpha-selection-intersect", n_layers > 0 && !fs && !ac);
 
 #undef SET_VISIBLE
 #undef SET_SENSITIVE
diff --git a/app/actions/layers-commands.c b/app/actions/layers-commands.c
index 68574743a1..ee67147f05 100644
--- a/app/actions/layers-commands.c
+++ b/app/actions/layers-commands.c
@@ -1555,14 +1555,23 @@ layers_alpha_to_selection_cmd_callback (GimpAction *action,
                                         gpointer    data)
 {
   GimpImage      *image;
-  GimpLayer      *layer;
+  GList          *layers;
+  GList          *iter;
   GimpChannelOps  operation;
-  return_if_no_layer (image, layer, data);
+  return_if_no_layers (image, layers, data);
 
   operation = (GimpChannelOps) g_variant_get_int32 (value);
 
-  gimp_item_to_selection (GIMP_ITEM (layer), operation,
-                          TRUE, FALSE, 0.0, 0.0);
+  for (iter = layers; iter; iter = iter->next)
+    {
+      if (operation != GIMP_CHANNEL_OP_REPLACE || iter == layers)
+        gimp_item_to_selection (GIMP_ITEM (iter->data), operation,
+                                TRUE, FALSE, 0.0, 0.0);
+      else
+         gimp_item_to_selection (GIMP_ITEM (iter->data),
+                                 GIMP_CHANNEL_OP_ADD,
+                                 TRUE, FALSE, 0.0, 0.0);
+    }
   gimp_image_flush (image);
 }