app: copy the buffer rather than using it as source.

I think the previous code should be OK, but I had some criticals when
painting with a paint tool in the area which got extended:

> (gimp:577203): GLib-GObject-CRITICAL **: 21:37:08.402: value "-31" of type 'gint' is invalid or out of range for property 'y' of type 'gint'

It looks like there lingering pieces from the negative offset in the
buffer, which is probably a bug in GEGL?
Anyway let's go the shorter route for now, which is to copy the buffer
with a different offset. I don't think it's less efficient either
anyway.

app/core/gimpdrawable.c

@@ -1041,11 +1041,13 @@ gimp_drawable_real_set_buffer (GimpDrawable        *drawable,
        * This may happen for instance when merging filters which may
        * render in negative coordinates.
        */
-      buffer = g_object_new (GEGL_TYPE_BUFFER,
-                             "source",  buffer,
-                             "shift-x", extent->x,
-                             "shift-y", extent->y,
-                             NULL);
+      GeglBuffer    *buffer2;
+      GeglRectangle  extent2 = { 0, 0, extent->width, extent->height };
+
+      buffer2 = gegl_buffer_new (&extent2, gegl_buffer_get_format (buffer));
+      gimp_gegl_buffer_copy (buffer, extent, GEGL_ABYSS_NONE, buffer2, &extent2);
+
+      buffer      = buffer2;
       free_buffer = TRUE;
     }