core: Fix Alpha to Selection on single no-alpha layer

Resolves #14392
When using Alpha to Selection on a single layer with no
transparency, we were not cropping the selection channel
from the image size to the the layer size. As a result, we were
essentially performing Select All instead of only selecting the
area of the layer itself.
This patch refactors gimp_channel_combine_items () in its
single layer check, to create a mask for single layers with no
alpha that is the size of the layer. It's then used to crop the
image channel the same way the alpha channel does for layers
that have transparency.

Author: cmyk-student

app/core/gimpchannel-combine.c

@@ -556,24 +556,28 @@ gimp_channel_combine_items (GimpChannel    *mask,
 
       if (g_list_length (layers) == 1)
         {
+          GimpChannel *alpha;
+          gint         offset_x;
+          gint         offset_y;
+
           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,
-                                         offset_x, offset_y);
-              g_object_unref (alpha);
             }
           else
             {
-              gimp_channel_all (channel, FALSE);
+              alpha = gimp_channel_new (image,
+                                        gimp_item_get_width (layers->data),
+                                        gimp_item_get_height (layers->data),
+                                        NULL, NULL);
+              gimp_channel_all (alpha, FALSE);
             }
+          gimp_item_get_offset (layers->data, &offset_x, &offset_y);
+          gimp_channel_combine_mask (channel, alpha,
+                                     GIMP_CHANNEL_OP_REPLACE,
+                                     offset_x, offset_y);
+          g_object_unref (alpha);
         }
     }