Less global variables: at any invocation of decoder_sleep(), we have a
reference to the DecoderControl anyway, so we should pass it.  This
costs less than having to call getPlayerData() in every tiny
function.  Maybe some day we will be able to have multiple decoders at
the same time...

git-svn-id: https://svn.musicpd.org/mpd/trunk@7316 09075e82-0dd4-0310-85a5-a0d7c8717e4f