mixer/All: auto-open "global" mixers

If a mixer is "global", it is available even if the output isn't
open.  However, since the check was changed from IsEnabled() to
IsReallyEnabled(), enabled outputs have not yet been used have not
been "really" enabled yet, preventing using the mixer.

Fixes a regression by commit 35dbc1a90c
(part of https://github.com/MusicPlayerDaemon/MPD/pull/1480).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1563

NEWS

@@ -5,6 +5,7 @@ ver 0.23.8 (not yet released)
   - pipewire: fix crash with PipeWire 0.3.53
 * mixer
   - better error messages
+  - alsa: fix setting volume before playback starts
 * support libfmt 9
 
 ver 0.23.7 (2022/05/09)

src/mixer/MixerAll.cxx

@@ -91,7 +91,10 @@ output_mixer_set_volume(AudioOutputControl &ao, unsigned volume)
 
 	/* software mixers are always updated, even if they are
 	   disabled */
-	if (!ao.IsReallyEnabled() && !mixer->IsPlugin(software_mixer_plugin))
+	if (!mixer->IsPlugin(software_mixer_plugin) &&
+	    /* "global" mixers can be used even if the output hasn't
+	       been used yet */
+	    !(mixer->IsGlobal() ? ao.IsEnabled() : ao.IsReallyEnabled()))
 		return SetVolumeResult::DISABLED;
 
 	try {

src/mixer/MixerInternal.hxx

@@ -63,6 +63,10 @@ public:
 		return &plugin == &other;
 	}
 
+	bool IsGlobal() const noexcept {
+		return plugin.global;
+	}
+
 	/**
 	 * Open mixer device
 	 *