mixer: added flag "open"

Remember if a mixer object is open or closed.  Don't call open() again
if it is already open.  This guarantees that the mixer plugin is
always called in a consistent state, and we will be able to remove
lots of checks from the implementations.

To support mixers which are automatically opened even if the audio
output is still closed (to set the volume before playback starts),
this patch also adds the "global" flag to the mixer_plugin struct.
Both ALSA and OSS set this flag, while PULSE does not.

src/mixer_control.h

@@ -46,6 +46,13 @@ mixer_open(struct mixer *mixer);
 void
 mixer_close(struct mixer *mixer);
 
+/**
+ * Close the mixer unless the plugin's "global" flag is set.  This is
+ * called when the #audio_output is closed.
+ */
+void
+mixer_auto_close(struct mixer *mixer);
+
 int
 mixer_get_volume(struct mixer *mixer);