vorbisdec, theoradec: don't push tags if demuxer has already extracted identical ones

Submitted by Tim Müller @tpm

Link to original bug (#638579)

Description

Now that oggdemux also reads tags from vorbis/theora/etc. vorbiscomment headers, vorbisdec etc. don't really need to parse those any more. If they do, they should probably not post them or forward them downstream, since that just leads to duplicate tags.

Wonder if we can just remove tag extraction here or if there's still a good reason to keep it in the decoder(s).