output_init: added option to disable the hardware mixer

doc/user.xml

@@ -285,6 +285,17 @@ cd mpd-0.14.2</programlisting>
                 enabled.
               </entry>
             </row>
+            <row>
+              <entry>
+                <varname>mixer_enabled</varname>
+                <parameter>yes|no</parameter>
+              </entry>
+              <entry>
+                Specifies whether the hardware mixer of this audio
+                output should be used.  By default, all hardware
+                mixers are enabled if available.
+              </entry>
+            </row>
           </tbody>
         </tgroup>
       </informaltable>

src/output_init.c

@@ -135,7 +135,8 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 	if (ao->data == NULL)
 		return false;
 
-	if (plugin->mixer_plugin != NULL)
+	if (plugin->mixer_plugin != NULL &&
+	    config_get_block_bool(param, "mixer_enabled", true))
 		ao->mixer = mixer_new(plugin->mixer_plugin, param);
 	else
 		ao->mixer = NULL;