Lot of changes here.

1. check_classic_mac_basic.yml — update to latest macOS-latest
2. docker-on-push-tag-or-pr — dependant updates
3. FFTConvolver — fix a compilation warning when the DEBUG flag is turned off
4. Reorganise some source files — e.g. move most of the metadata stuff into a subdirectory
5. Remove unused special-purpose AirPlay 2 RC event handler.
6. Reorganise source files and source code relating to the standard event handler.
7. Check configuration settings a little more carefully to offer more meaningful diagnostics.
8. Add password to AirPlay 2 and reorganise initialisation to set AirPlay 2 flags accordingly.  Also, remove redundant airplay_pin.
9. Remove redundant debug facility definitions from common.h
10. Make it possible to omit all the dacp stuff from a build.
11. Make it possible to omit metadata multicast from a build.
12. Add AP2-native metadata by setting bit 50 of features to true.
13. Improve stability when pre-empting an existing play session.
14. Fix a few compiler warnings in the pair_ap library.
15. Ensure that the audio format is known before starting synchronisation.
16. Improve handle_configure to set/clear a new config setting. Doesn’t seem to do anything…
17. Experimentally turn off bit 46 of AirPlay 2 features — seems to stop failing pair-verify attempts.

metadata/pc_queue.h

@@ -0,0 +1,26 @@
+
+#pragma once
+
+#include "common.h"
+#include "config.h"
+#include <pthread.h>
+
+typedef struct {
+  pthread_mutex_t pc_queue_lock;
+  pthread_cond_t pc_queue_item_added_signal;
+  pthread_cond_t pc_queue_item_removed_signal;
+  char *name;
+  size_t item_size;  // number of bytes in each item
+  uint32_t count;    // number of items in the queue
+  uint32_t capacity; // maximum number of items
+  uint32_t toq;      // first item to take
+  uint32_t eoq;      // free space at end of queue
+  void *items;       // a pointer to where the items are actually stored
+} pc_queue;          // producer-consumer queue
+
+void pc_queue_init(pc_queue *the_queue, char *items, size_t item_size, uint32_t number_of_items,
+                   const char *name);
+
+void pc_queue_delete(pc_queue *the_queue);
+int pc_queue_add_item(pc_queue *the_queue, const void *the_stuff, int block);
+int pc_queue_get_item(pc_queue *the_queue, void *the_stuff);