gstavviddec: Only allow a single closed caption meta

gstavviddec: Only allow a single closed caption meta

Following discussion in
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1396#note_556068

While it is technically possible to store multiple closed caption metas
in the same buffer, we don't currently do that anywhere and for
H264/MPEG2 both parts have to be stored in the same packet, and also the
number of CC bytes you can store per frame is rather limited. This
restriction might be relaxed later once we figured out how to do it
without breaking things.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-libav/-/merge_requests/82>