WebVTT subtitles in WebM require special logic, and we signal that using
the "webvtt-webm" codec name, which is not part of FFmpeg. That worked
fine until commit 94859997b8 ("sub: update codec info") added a call
to mp_codec_info_from_av(), which overwrites our codec name with
FFmpeg's the first time the subtitle decoder is initialized. As a
result, WebM WebVTT subtitles are currently not rendered correctly after
switching subtitle tracks. Fix the issue by resetting the codec name
after calling mp_codec_info_from_av().
Thomas Hebb
036fae317c