app: add gimp_display_shell_get_canvas_pickable()

... which is similar to gimp_display_shell_get_pickable(), however,
it returns the projection, rather than the image, only when
gimp_display_shell_get_infinite_canvas() is TRUE, i.e., when the
shell is in "show all" mode *and* canvas padding is disabled.

(cherry picked from commit 71f42f6675)

app/display/gimpdisplayshell.c

@@ -1873,6 +1873,26 @@ gimp_display_shell_get_pickable (GimpDisplayShell *shell)
   return NULL;
 }
 
+GimpPickable *
+gimp_display_shell_get_canvas_pickable (GimpDisplayShell *shell)
+{
+  GimpImage *image;
+
+  g_return_val_if_fail (GIMP_IS_DISPLAY_SHELL (shell), NULL);
+
+  image = gimp_display_get_image (shell->display);
+
+  if (image)
+    {
+      if (! gimp_display_shell_get_infinite_canvas (shell))
+        return GIMP_PICKABLE (image);
+      else
+        return GIMP_PICKABLE (gimp_image_get_projection (image));
+    }
+
+  return NULL;
+}
+
 GeglRectangle
 gimp_display_shell_get_bounding_box (GimpDisplayShell *shell)
 {

app/display/gimpdisplayshell.h

@@ -310,6 +310,8 @@ void              gimp_display_shell_set_show_all  (GimpDisplayShell   *shell,
                                                     gboolean            show_all);
 
 GimpPickable    * gimp_display_shell_get_pickable  (GimpDisplayShell   *shell);
+GimpPickable    * gimp_display_shell_get_canvas_pickable
+                                                   (GimpDisplayShell   *shell);
 GeglRectangle     gimp_display_shell_get_bounding_box
                                                    (GimpDisplayShell   *shell);
 gboolean          gimp_display_shell_get_infinite_canvas