mixer: protect the mixer struct with a mutex

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.

src/mixer_api.h

@@ -23,8 +23,16 @@
 #include "mixer_plugin.h"
 #include "mixer_list.h"
 
+#include <glib.h>
+
 struct mixer {
 	const struct mixer_plugin *plugin;
+
+	/**
+	 * This mutex protects all of the mixer struct, including its
+	 * implementation, so plugins don't have to deal with that.
+	 */
+	GMutex *mutex;
 };
 
 void