qtdemux: support timed-text subtitle tracks.

https://bugzilla.gnome.org/show_bug.cgi?id=752818

gst/isomp4/fourcc.h

@@ -195,6 +195,7 @@ G_BEGIN_DECLS
 #define FOURCC_sowt     GST_MAKE_FOURCC('s','o','w','t')
 #define FOURCC_stbl     GST_MAKE_FOURCC('s','t','b','l')
 #define FOURCC_stco     GST_MAKE_FOURCC('s','t','c','o')
+#define FOURCC_stpp     GST_MAKE_FOURCC('s','t','p','p')
 #define FOURCC_stps     GST_MAKE_FOURCC('s','t','p','s')
 #define FOURCC_strf     GST_MAKE_FOURCC('s','t','r','f')
 #define FOURCC_strm     GST_MAKE_FOURCC('s','t','r','m')

gst/isomp4/qtdemux.c

@@ -12684,6 +12684,10 @@ qtdemux_sub_caps (GstQTDemux * qtdemux, QtDemuxStream * stream,
       /* actual text piece needs to be extracted */
       stream->need_process = TRUE;
       break;
+    case FOURCC_stpp:
+      _codec ("XML subtitles");
+      caps = gst_caps_new_empty_simple ("application/ttml+xml");
+      break;
     default:
     {
       char *s, fourstr[5];

gst/isomp4/qtdemux_types.c

@@ -201,6 +201,7 @@ static const QtNodeType qt_node_types[] = {
   {FOURCC_schi, "scheme information", QT_FLAG_CONTAINER},
   {FOURCC_pssh, "protection system specific header", 0},
   {FOURCC_tenc, "track encryption", 0},
+  {FOURCC_stpp, "XML subtitle sample entry", 0},
   {0, "unknown", 0,},
 };