Preserve user and application XMP metadata across encodes (JPEG/HEIC/AVIF)

[DichenZhang1]

Feature Description

Currently, libultrahdr provides public APIs and passthrough logic for EXIF (uhdr_enc_set_exif_data, uhdr_dec_get_exif) and ICC profiles (uhdr_dec_get_icc), but does not preserve or expose general user/application XMP metadata across encode and transcode pipelines.

In professional photography workflows (e.g., Adobe Lightroom, Camera Raw, Photoshop, Lumenzia), source images frequently carry rich XMP metadata packets containing:

• Copyright and author attribution (Dublin Core)
• Non-destructive develop settings (crs: adjustments, tone curves, masks, color grading)
• Asset management tags (ratings, color labels, keywords, GPS tags)

Current Behavior

• JPEG (jpegr.cpp): When reordering base JPEG markers in appendGainMap, all APP markers (0xE0–0xEF) from the base JPEG are skipped, silently stripping existing APP1 XMP packets. If kWriteXmpMetadata is enabled, a brand-new XMP packet is synthesized containing only the UltraHDR container directory (Container:Directory), wiping out original user XMP.
• HEIC & AVIF (heifultrahdr.cpp, avifultrahdr.cpp): Only EXIF metadata is added to the primary image handle; XMP is neither written during encoding nor extracted during decoding.
• API: ultrahdr_api.h lacks setter/getter APIs for raw XMP blocks (e.g., uhdr_enc_set_xmp_data / uhdr_dec_get_xmp).

Proposed Solution

1. Public API:
   • Add uhdr_enc_set_xmp_data(uhdr_codec_private_t* enc, uhdr_mem_block_t* xmp)
   • Add uhdr_dec_get_xmp(uhdr_codec_private_t* dec)
   • Expose corresponding flags in ultrahdr_app CLI.
2. HEIC / AVIF Support:
   • In HeifUltraHdr and AvifUltraHdr, attach user XMP via libheif (heif_context_add_xmp_metadata) and extract via heif_image_handle_get_list_of_metadata_block_IDs. Because gain maps in HEIC/AVIF use ISO 21496-1 tmap items, no XML manipulation is required.
3. JPEG Support:
   • ISO 21496-1 Mode: Preserve the existing APP1 XMP marker untouched when writing ISO gain map metadata in APP2.
   • XMP / Dual Mode: Merge the UltraHDR container RDF tags into the existing XMP <x:xmpmeta> document, handling Extended XMP segmentation if the combined packet exceeds the 64KB JPEG segment limit.