[idd] scheduler: retain frames for publication deadlines

Prepare each acquired content frame into one of two private GPU
candidates so the IddCx frame can be released without copying it to
IVSHMEM immediately.

Wake a high-resolution publisher at the client deadline and send the
newest completed candidate. Preserve accumulated damage across
superseded candidates, include the scheduling hold in end-to-end timing,
and keep transport work on the existing physical copy queue.

idd/LGIdd/CFrameBufferResource.h

@@ -43,6 +43,7 @@ class CFrameBufferResource
     unsigned               m_timingEffectIndex = 0;
     uint64_t               m_timingToken       = 0;
     bool                   m_fullCopy          = false;
+    unsigned               m_candidateIndex    = 0;
     ComPtr<ID3D12Resource> m_res;
     void                 * m_map               = nullptr;
 
@@ -75,6 +76,8 @@ class CFrameBufferResource
     unsigned GetTimingEffectIndex() const { return m_timingEffectIndex; }
     uint64_t GetTimingToken      () const { return m_timingToken;       }
     bool     IsFullCopy          () const { return m_fullCopy;          }
+    void     SetCandidateIndex(unsigned index) { m_candidateIndex = index; }
+    unsigned GetCandidateIndex() const         { return m_candidateIndex; }
 
     ComPtr<ID3D12Resource> Get() { return m_res; }
 };