The ogg/opus tagger.

The ogg/opus tagger.

This adds support for modifying meta tags of ogg/opus files. The heart
of this patch is ogg_rewrite_tags(), a codec-independent function
which replaces the meta packet of an ogg stream. This function will
also be used in subsequent patches which add support for ogg/vorbis
and ogg/speex files.

In order to create identically sized ogg pages for the output, the
new ogg_rewrite_tags() function calls ogg_stream_flush_fill() if it
is available. This function was introduced in libogg version 1.3.0.
So this commit adds a configure check and makes ogg_rewrite_tags()
fall back to ogg_stream_flush() on systems where libogg lacks
ogg_stream_flush_fill().