[idd] capture: keep preparing during publication

Do not freeze the spare retained candidate while a frame is copied to
IVSHMEM. Allow newer source frames to replace that candidate and retain
any frame prepared after the published sequence for the next deadline.

Take the pipeline lock before selecting the publication candidate so its
transport copy is submitted before any newly admitted preparation.

idd/LGIdd/CSwapChainProcessor.h

@@ -62,7 +62,6 @@ private:
     CANDIDATE_PREPARING,
     CANDIDATE_READY,
     CANDIDATE_PUBLISHING,
-    CANDIDATE_HELD,
   };
 
   struct FrameCandidate