[idd] capture: retain frames in linear buffers

Store retained frame candidates in the final linear transport layout.
Prepare texture output in local DEFAULT buffers before cadence holding.
Then publish full frames with a single buffer copy.

Keep partial damage on matching pitched layouts and leave effect-owned
buffer copies unchanged. This moves detiling out of the
deadline-critical IVSHMEM or readback transfer.

idd/LGIdd/CPostProcessor.h

@@ -183,7 +183,10 @@ public:
   size_t   GetOutputSize () const { return m_frameSize; }
   bool ShouldCopyFully(
     const RECT dirtyRects[], unsigned nbDirtyRects) const;
-  void CopyFrame(
+  void CopyToCandidate(
+    const ComPtr<ID3D12GraphicsCommandList>& commandList,
+    ID3D12Resource * dst, ID3D12Resource * src) const;
+  void CopyFromCandidate(
     const ComPtr<ID3D12GraphicsCommandList>& commandList,
     ID3D12Resource * dst, ID3D12Resource * src,
     const RECT dirtyRects[], unsigned nbDirtyRects, bool fullCopy) const;