Given buffer could be fragmented and we might need to collect buffers until end tag is detected. And/or, a buffer can consist of multiple ttml documents. Fixes: https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/494
Add a parser (ttmlparse) and renderer (ttmlrender) element that handle subtitles that use the EBU-TT-D profile of TTML1. https://bugzilla.gnome.org/show_bug.cgi?id=758232