[gimp] app: add "show all" support to the Clone tool

[Ell <ell src gnome org>]

commit c5bf595fe6bf3dc8a82e5c59cb205d52d736ee02
Author: Ell <ell_se yahoo com>
Date:   Fri Sep 6 20:03:31 2019 +0300

    app: add "show all" support to the Clone tool
    
    ... by using gimp_paint_core_get_image_pickable() in
    GimpSourceCore, and adapting it to handle source buffers with
    arbitrary extents.

 app/paint/gimpsourcecore.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/app/paint/gimpsourcecore.c b/app/paint/gimpsourcecore.c
index 02db2186f6..f0ca584215 100644
--- a/app/paint/gimpsourcecore.c
+++ b/app/paint/gimpsourcecore.c
@@ -408,10 +408,9 @@ gimp_source_core_motion (GimpSourceCore   *source_core,
 
       if (options->sample_merged)
         {
-          GimpImage *src_image = gimp_pickable_get_image (src_pickable);
-          gint       off_x, off_y;
+          gint off_x, off_y;
 
-          src_pickable = GIMP_PICKABLE (src_image);
+          src_pickable = gimp_paint_core_get_image_pickable (paint_core);
 
           gimp_item_get_offset (GIMP_ITEM (source_core->src_drawable),
                                 &off_x, &off_y);
@@ -594,7 +593,8 @@ gimp_source_core_real_get_source (GimpSourceCore   *source_core,
                                   paint_buffer_y + src_offset_y,
                                   gegl_buffer_get_width  (paint_buffer),
                                   gegl_buffer_get_height (paint_buffer),
-                                  0, 0,
+                                  gegl_buffer_get_x      (src_buffer),
+                                  gegl_buffer_get_y      (src_buffer),
                                   gegl_buffer_get_width  (src_buffer),
                                   gegl_buffer_get_height (src_buffer),
                                   &x, &y,