ceaccoverlay: New CEA-708 Closed Caption decoder and overlayer

ceaccoverlay: New CEA-708 Closed Caption decoder and overlayer

This new element allows decoding and overlaying CEA-708 Closed Caption
streams over video.

* Supports CDP and cc_data closedcaption/x-cea-708 streams
* Uses pango to render CC stream
* Support GstVideoOverlayComposition meta if downstream supports is

Tested on various test files.

Remains to be fixed/improved:
* Switch to GstByteReader (for code safety)
* Switch to GString (instead of manual pango string construction)
* Move pango/rendering code outside of main 708 decoder file (so
  that actual CC parser/decoder can be (re)used in other scenarios).

Initial patches and improvements by:
* CableLabs RUIH-RI Team <ruihri@cablelabs.com>
* Steve Maynard <steve@secondstryke.com>
* cjun.wang" <cjun.wang@samsung.com>

https://bugzilla.gnome.org/show_bug.cgi?id=704881