ext/pango/gsttextoverlay.*: If input is plain text, escape it before passing it to pango_layout_set_markup().

Original commit message from CVS:
* ext/pango/gsttextoverlay.c: (gst_text_overlay_init),
(gst_text_overlay_setcaps_txt), (gst_text_overlay_video_chain):
* ext/pango/gsttextoverlay.h:
If input is plain text, escape it before passing it to
pango_layout_set_markup().