As a side effect, the previous patch added the mixer==NULL checks. It
is now illegal to call mixer functions with a NULL argument. Convert
the runtime checks to assertions.
In some rare cases, there was a race condition between the output
thread and the main thread: when you disable/enable an output device
in the main thread, this caused a crash in the output thread. Protect
the whole mixer struct with a GMutex to prevent that.
This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.
