app: fix GEGL buffer leaks.

Since commit 10b96b1025, we had buffer leaks as soon as we ran filters.
gimp_drawable_get_buffer_with_effects() was either creating a new buffer (when a
drawable had filters) or not. And calling code couldn't know which is which.

Code needs to be consistent. Either we return a new reference to take ownership
to, or not. So let's change to add a new reference to the drawable' buffers so
that it always returns a new reference to free.

app/core/gimpdrawable-preview.c

@@ -83,7 +83,8 @@ sub_preview_data_new (const Babl          *format,
   SubPreviewData *data = g_slice_new (SubPreviewData);
 
   data->format = format;
-  data->buffer = g_object_ref (buffer);
+  /* We take ownership if the buffer reference. */
+  data->buffer = buffer;
   data->rect   = *rect;
   data->scale  = scale;
 
@@ -231,6 +232,7 @@ gimp_drawable_get_sub_preview (GimpDrawable *drawable,
                    gimp_temp_buf_get_format (preview),
                    gimp_temp_buf_get_data (preview),
                    GEGL_AUTO_ROWSTRIDE, GEGL_ABYSS_CLAMP);
+  g_object_unref (buffer);
 
   return preview;
 }
@@ -328,6 +330,8 @@ gimp_drawable_get_sub_pixbuf (GimpDrawable *drawable,
                        GEGL_ABYSS_CLAMP);
     }
 
+  g_object_unref (buffer);
+
   return pixbuf;
 }
 

app/core/gimpdrawable.c

@@ -1537,17 +1537,17 @@ gimp_drawable_get_buffer_with_effects (GimpDrawable *drawable)
             }
           else
             {
-              return buffer;
+              return g_object_ref (buffer);
             }
         }
       else
         {
-          return GIMP_DRAWABLE_GET_CLASS (drawable)->get_buffer (drawable);
+          return g_object_ref (GIMP_DRAWABLE_GET_CLASS (drawable)->get_buffer (drawable));
         }
     }
   else
     {
-      return drawable->private->paint_buffer;
+      return g_object_ref (drawable->private->paint_buffer);
     }
 }
 

app/core/gimppickable.c

@@ -200,7 +200,7 @@ gimp_pickable_get_buffer_with_effects (GimpPickable *pickable)
   if (pickable_iface->get_buffer_with_effects)
     return pickable_iface->get_buffer_with_effects (pickable);
   else if (pickable_iface->get_buffer)
-    return pickable_iface->get_buffer (pickable);
+    return g_object_ref (pickable_iface->get_buffer (pickable));
 
   return NULL;
 }