Decode HDR video into the linear compositor correctly instead of approximating
everything as sRGB/BT.709:
- Read the frame's color_trc and color_primaries in the importer → VideoTransfer
{Gamma,Pq,Hlg} + VideoPrimaries {Bt709,Bt2020} on GpuVideoFrame.
- nv12_blit.wgsl: branch the EOTF — sRGB gamma (SDR), SMPTE2084 PQ (normalized so
203-nit graphics white = 1.0; highlights exceed 1.0), or HLG inverse-OETF
(reference white ≈ 1.0). Then BT.2020→BT.709 primaries in linear light when
wide-gamut, clamping out-of-709 colours.
Establishes the white=1.0 scene-linear convention: SDR content is unchanged
(stays in [0,1]); HDR video carries super-white highlights through compositing.
SDR-output mapping (clip default vs highlight rolloff) is Part 2. HLG's display
OOTF is omitted (scene-referred) — approximate but reasonable for SDR-out.
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