plugin: Do not use weak ref on importer

There is no good reason to keep using weak ref on importer within paintable object. Change that to a gst_object_ref which is faster.

lib/gst/plugin/gstclapperpaintable.h

@@ -46,7 +46,8 @@ struct _GstClapperPaintable
 
   GdkRGBA bg;
 
-  GWeakRef widget, importer;
+  GWeakRef widget;
+  GstClapperImporter *importer;
 
   /* Sink properties */
   gint par_n, par_d;