splitmuxsink: Add option for timecode-based split

If this option is given, it will calculate the next split point based on
timecode difference.

https://bugzilla.gnome.org/show_bug.cgi?id=774209

gst/multifile/gstsplitmuxsink.h

@@ -89,13 +89,15 @@ typedef struct _MqStreamCtx
   GstClockTimeDiff in_running_time;
   GstClockTimeDiff out_running_time;
 
+  GstBuffer *prev_in_keyframe; /* store keyframe for each GOP */
+
   GstElement *q;
   GQueue queued_bufs;
 
   GstPad *sinkpad;
   GstPad *srcpad;
 
-  GstBuffer *cur_buffer;
+  GstBuffer *cur_out_buffer;
   GstEvent *pending_gap;
 } MqStreamCtx;
 
@@ -113,6 +115,8 @@ struct _GstSplitMuxSink
   guint64 threshold_bytes;
   guint max_files;
   gboolean send_keyframe_requests;
+  gchar *threshold_timecode_str;
+  GstClockTime next_max_tc_time;
 
   GstElement *muxer;
   GstElement *sink;