app: use gimp_async_add_callback_for_object() in various places

Use gimp_async_add_callback_for_object(), added in the previous
commit, instead of gimp_async_add_callback(), in cases where the
destructor of the object owning the async doesn't wait for the
async to finish.  This avoids leaking such ongoing asyncs on
shutdown, during which gimp-parallel either finishes or aborts the
asyncs: if at this point an async has any registered callbacks, an
idle source is added for running the callbacks, extending the
lifetime of the async; however, since we're not getting back into
the main loop, the idle is never run, and the async (and any
associated resources) are never freed.

app/text/gimpfontfactory.c

@@ -336,9 +336,11 @@ gimp_font_factory_load (GimpFontFactory  *factory,
     (GimpParallelRunAsyncFunc) gimp_font_factory_load_async,
     config);
 
-  gimp_async_add_callback (async,
-                           (GimpAsyncCallback) gimp_font_factory_load_async_callback,
-                           factory);
+  gimp_async_add_callback_for_object (
+    async,
+    (GimpAsyncCallback) gimp_font_factory_load_async_callback,
+    factory,
+    factory);
 
   gimp_async_set_add (async_set, async);
 

app/widgets/gimpviewrendererdrawable.c

@@ -308,9 +308,10 @@ gimp_view_renderer_drawable_render (GimpViewRenderer *renderer,
       renderdrawable->priv->render_buf_y  = dst_y;
       renderdrawable->priv->render_update = FALSE;
 
-      gimp_async_add_callback (
+      gimp_async_add_callback_for_object (
         async,
         (GimpAsyncCallback) gimp_view_renderer_drawable_render_async_callback,
+        renderdrawable,
         renderdrawable);
 
       /* if rendering isn't done yet, update the render-view once it is, and