Drop the whitelist. It's annoying to maintain. Instead, accept any
subtitle decoder. Since this code path will now also be taken by bitmap
subtitle decoders not whitelisted by sd_lavc.c, add a warning when
bitmap subtitles are decoded. (To reduce or increase potential user
confusion.)
To some degree, a whitelist is needed to distinguish text and bitmap
subtitles. FFmpeg has an API to distinguish them in a generic way to
some degree, but Libav doesn't. So we just stick with this for now.
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