Don't add it to the filter chain, because we need to apply replay gain
before cross-fading with the next song.  Add a second replay_gain
filter which is used for the song being faded in (chunk->other).

Remove cross_fade_apply(), and call pcm_mix() in the output thread,
mixing the chunk and chunk->other together.

Preparation for cross-fading support in the output thread.

Move encoder before audio outputs. Makes sense to keep them after
decoders and some audio outputs need to know wheather encoders are
present.

Add libogg section rather than checking for it every time it's necessary.

Add optional third value to results() to allow for non "yes" answers which are
considered positive.

Rewrite the liboggflac tests, the m4 macro is way too complicated for no
reason and it's buggy.

This commit adds a new pretty print menu, we've (far) outgrown the old
menu which because hard to traverse.

Ogg Tremor doesn't have the ability to be disabled under the first if
conditional, might as well AC_DEFINE there.