output: use the software mixer plugin

Do all the software volume stuff inside each output thread, not in the
player thread.  This allows one software mixer per output device, and
also allows the user to configure the mixer type (hardware or
software) for each audio output.

This moves the global "mixer_type" setting into the "audio_output"
section, deprecating the "mixer_enabled" flag.

src/mixer_control.h

@@ -31,9 +31,6 @@ struct mixer;
 struct mixer_plugin;
 struct config_param;
 
-void
-mixer_disable_all(void);
-
 struct mixer *
 mixer_new(const struct mixer_plugin *plugin, const struct config_param *param);