decoderParent() uses a lot of OutputBuffer internals to see whether
cross-fading should be started.  Move these checks to outputBuffer.c,
which also simplifies decoderParent().

git-svn-id: https://svn.musicpd.org/mpd/trunk@7262 09075e82-0dd4-0310-85a5-a0d7c8717e4f