Signal sRGB correctly in HEIF and AVIF NCLX profiles

[gregbenz]

Signal sRGB correctly in HEIF and AVIF NCLX profiles

Summary

• signal IEC 61966-2-1 for sRGB primary images in HEIF and AVIF output
• continue interpreting the BT.709 transfer value written by older libultrahdr versions as sRGB
• verify the serialized primary-image NCLX profile for both codecs

Why

The HEIF/AVIF encoding APIs require the SDR intent to use UHDR_CT_SRGB, but the output NCLX profile currently identifies that transfer as BT.709. This conflicts with the sRGB pixel encoding and the sRGB ICC profile written by the same path.

Viewers that honor NCLX may therefore select the BT.709 inverse transfer function for sRGB values, producing incorrect linear-light values that are most noticeable in shadows. The encoded pixels are unchanged by this fix; only their transfer-characteristic signaling is corrected.

The decoder retains BT.709 as a legacy alias for sRGB so files written by existing libultrahdr releases remain compatible.

Testing

• generated API-1 HEIF and AVIF images and inspected the serialized primary-image NCLX profiles
• confirmed both tests report BT.709 before the fix and IEC 61966-2-1 afterward
• complete unit suite passes

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