ShellRecorderSrc: Set as GST_FORMAT_TIME

We need to indicate that our GStreamer source produces timestamped
frames, instead of the default, which is to produce a stream of bytes.
This is needed for correctness, and to avoid warnings for some
pipelines.

https://bugzilla.gnome.org/show_bug.cgi?id=669066

src/shell-recorder-src.c

@@ -40,6 +40,8 @@ static void
 shell_recorder_src_init (ShellRecorderSrc      *src,
 			 ShellRecorderSrcClass *klass)
 {
+  gst_base_src_set_format (GST_BASE_SRC (src), GST_FORMAT_TIME);
+
   src->queue = g_async_queue_new ();
   src->mutex = &src->mutex_data;
   g_mutex_init (src->mutex);