The NDI closed captions specifications [1] define a variation where metadata is
attached to the video frame. This requires the AFD buffer to be v210 encoded.
This commit applies this strategy.
Another difference with previous version is that when an error occurs while
encoding or decoding a meta, next meta are also tried instead of failing
immediately.
Receiving closed captions as a standalone metadata is kept for interoperability
purposes. In this case, metadata is also expected to be v210 encoded.
[1]: http://www.sienna-tv.com/ndi/ndiclosedcaptions.html
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1356>