[client] audio: add transport provider interface

Route audio through a provider interface. Connected transports take
priority while SPICE remains the fallback.

Carry explicit sample formats, rates, channel layouts, and source clocks
through playback and recording. Add optional presentation clock feedback
for active synchronization.

client/include/interface/transport.h

@@ -27,6 +27,7 @@
 
 #include "common/framebuffer.h"
 #include "common/types.h"
+#include "interface/audio.h"
 #include "interface/input.h"
 
 #define LG_TRANSPORT_MAX_DAMAGE_RECTS LG_MAX_FRAME_DAMAGE_RECTS
@@ -250,6 +251,10 @@ typedef struct LG_TransportOps
    * valid until disconnect; NULL indicates that this session has no input. */
   const LG_InputOps *(*getInputOps)(LG_Transport * transport,
       void ** opaque);
+  /* Queried after connect. The returned operations and opaque value remain
+   * valid until disconnect; NULL indicates that this session has no audio. */
+  const LG_AudioOps *(*getAudioOps)(LG_Transport * transport,
+      void ** opaque);
   bool (*attachRenderer)(LG_Transport * transport,
       const LG_RendererInterop * interop);
   void (*detachRenderer)(LG_Transport * transport);