[idd] transport: track asynchronous frame readiness

idd/LGIdd/transport/PreparedFrameBuffer.h

@@ -33,12 +33,26 @@ enum : unsigned
 
 struct FrameToken
 {
-  uint32_t sink   = 0;
+  uint32_t backend = 0;
   uint32_t epoch  = 0;
   uint32_t slot   = 0;
   uint64_t serial = 0;
 };
 
+enum class FrameFill
+{
+  READY,
+  PENDING,
+  REJECTED,
+};
+
+enum class FrameDone
+{
+  READY,
+  FAILED,
+  SUPERSEDED,
+};
+
 struct PreparedFrameBuffer
 {
   FrameToken token;