[gimp] app: fix alpha to selection on single layer with offset.

[Jehan <jehanp src gnome org>]

commit 1a90a2df3271d7d45477ed3b8a4641aa969abd0d
Author: Jehan <jehan girinstud io>
Date:   Wed Oct 21 17:36:58 2020 +0200

    app: fix alpha to selection on single layer with offset.

 app/core/gimpchannel-combine.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/app/core/gimpchannel-combine.c b/app/core/gimpchannel-combine.c
index db8ccab7c9..db2a428abf 100644
--- a/app/core/gimpchannel-combine.c
+++ b/app/core/gimpchannel-combine.c
@@ -559,11 +559,15 @@ gimp_channel_combine_items (GimpChannel    *mask,
           if (gimp_drawable_has_alpha (layers->data))
             {
               GimpChannel *alpha;
+              gint         offset_x;
+              gint         offset_y;
 
               alpha = gimp_channel_new_from_alpha (image,
                                                    layers->data, NULL, NULL);
+              gimp_item_get_offset (layers->data, &offset_x, &offset_y);
               gimp_channel_combine_mask (channel, alpha,
-                                         GIMP_CHANNEL_OP_REPLACE, 0, 0);
+                                         GIMP_CHANNEL_OP_REPLACE,
+                                         offset_x, offset_y);
               g_object_unref (alpha);
             }
           else