audio_output: added function audio_output_is_pending()

The function audio_output_is_pending() returns whether there is a
pending command.  This is useful for output plugins as a break
condition for longer loops.

src/output_api.c

@@ -31,3 +31,7 @@ void audio_output_closed(struct audio_output *ao)
 	ao->open = 0;
 }
 
+bool audio_output_is_pending(const struct audio_output *ao)
+{
+	return ao->command != AO_COMMAND_NONE;
+}

src/output_api.h

@@ -27,6 +27,8 @@
 #include "log.h"
 #include "os_compat.h"
 
+#include <stdbool.h>
+
 #define DISABLED_AUDIO_OUTPUT_PLUGIN(plugin) const struct audio_output_plugin plugin;
 
 struct audio_output;
@@ -113,4 +115,9 @@ const char *audio_output_get_name(const struct audio_output *ao);
 
 void audio_output_closed(struct audio_output *ao);
 
+/**
+ * Returns true if there is a command pending.
+ */
+bool audio_output_is_pending(const struct audio_output *ao);
+
 #endif