Issue #440 - libgimp/gimptilebackendplugin.c provides no pyramid

In GimpTileBackendPlugin, return NULL when fetching z>0 tiles,
instead of simply ignoring the z coordinate, so that the mipmapped
tile is rendered locally.  Likewise, avoid storing z>0 tiles.

Note that this is suboptimal, since all the necessary level-0 tiles
need to be sent to the buffer as a result.  Ideally, we should
extend the wire protocol to handle mipmapped tiles.

libgimp/gimptilebackendplugin.c

@@ -132,20 +132,31 @@ gimp_tile_backend_plugin_command (GeglTileSource  *tile_store,
   switch (command)
     {
     case GEGL_TILE_GET:
+      /* TODO: fetch mipmapped tiles directly from gimp, instead of returning
+       * NULL to render them locally
+       */
+      if (z == 0)
+        {
           g_mutex_lock (&backend_plugin_mutex);
 
           result = gimp_tile_read_mul (backend_plugin, x, y);
 
           g_mutex_unlock (&backend_plugin_mutex);
+        }
       break;
 
     case GEGL_TILE_SET:
+      /* TODO: actually store mipmapped tiles */
+      if (z == 0)
+        {
           g_mutex_lock (&backend_plugin_mutex);
 
           gimp_tile_write_mul (backend_plugin, x, y, gegl_tile_get_data (data));
-      gegl_tile_mark_as_stored (data);
 
           g_mutex_unlock (&backend_plugin_mutex);
+        }
+
+      gegl_tile_mark_as_stored (data);
       break;
 
     case GEGL_TILE_FLUSH: