matroskademux: emits invalid Pango markup

Submitted by Philippe Normand @philn

Link to original bug (#795891)

Description

VLC doesn't complain with that broken markup. Maybe it's actually valid? For WebVTT I think it is, at least.

0:00:15.766899000 26502 0x7f995da8fc50 DEBUG pango gstbasetextoverlay.c:2250:void gst_base_text_overlay_render_text(GstBaseTextOverlay *, const gchar *, gint): Rendering '<i>I'll try to contact them...'

(<unknown>:26502): Pango-WARNING **: 21:39:58.241: pango_layout_set_markup_with_accel: Error on line 1 char 47: Element 'markup' was closed, but the currently open element is 'i'

This isn't rendered. Instead the most recent valid text is rendered, which is confusing :)