[host] hdr: encode legacy captures as HDR10

Convert FP16 scRGB captures to BT.2020 PQ before storing them in the
10-bit wire format. Preserve linear-light downsampling, publish the real
SDR white level and display metadata, and advance the format version when
those properties change.

host/platform/Windows/capture/D12/d12.h

@@ -52,6 +52,8 @@ typedef struct D12FrameFormat
   DXGI_COLOR_SPACE_TYPE colorSpace;
   unsigned              width, height;
   CaptureFormat         format;
+  bool                  hdr;
+  bool                  hdrPQ;
 }
 D12FrameFormat;