imlib.c: fix support for alpha channel when saving using Imlib2.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@3350 92316355-f0b4-4df1-b90c-862c8a59935f

pipi/codec/imlib.c

@@ -73,6 +73,7 @@ int pipi_save_imlib2(pipi_image_t *img, const char *name)
         void *data;
 
         imlib_context_set_image(priv);
+        imlib_image_set_has_alpha(1);
         data = imlib_image_get_data();
 
         /* FIXME: check pitch differences here */