Misc AVIF fixes/improvements

[bep]

• Misc
• Fix AVIF decoding iloc/iinf ordering issue
• Speed up AVIF and TIFF decoding when only config source is requested

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.25%. Comparing base (0985245) to head (368fcfc).

Files with missing lines	Patch %	Lines
imagedecoder_webp.go	71.42%	0 Missing and 2 partials ⚠️
imagedecoder_heif.go	96.55%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   78.19%   78.25%   +0.05%     
==========================================
  Files          15       15              
  Lines        1885     1890       +5     
==========================================
+ Hits         1474     1479       +5     
  Misses        279      279              
  Partials      132      132              

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[Copilot]

Pull request overview

This PR improves AVIF/HEIF metadata decoding robustness and speeds up config-only decoding paths, adding new AVIF fixtures to cover the ordering edge case.

Changes:

• Fix HEIF/AVIF iloc/iinf box ordering sensitivity by collecting iloc entries keyed by item ID and resolving after the full meta scan.
• Speed up config-only decoding for TIFF (early-return after reading dimensions) and for AVIF/HEIF (skip iinf/iloc parsing when EXIF/XMP isn’t requested).
• Add AVIF test images plus EXIFTool “golden” JSON/config outputs, and add a focused AVIF decode test + benchmarks for config-only paths.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/images/bep/sunset.avif	New AVIF fixture used to validate EXIF/XMP extraction (and ordering edge case).
testdata/images/bep/sunrise.avif	New AVIF fixture used for benchmarks and golden comparisons.
imagemeta_test.go	Adds TestDecodeAVIF, enables .avif in getSunrise, and adds config/* benchmark cases + AVIF benchmarks.
imagedecoder_webp.go	Refactors WebP FourCC constants into a grouped webpFCC struct (no logic change).
imagedecoder_tif.go	Returns early when Sources == CONFIG after reading TIFF dimensions.
imagedecoder_heif.go	Makes EXIF/XMP extraction independent of iloc/iinf ordering; skips EXIF/XMP parsing work when not needed; refactors FourCC constants into heifFCC.
gen/testdata_exiftool/images/bep/sunset.avif.json	Adds EXIFTool golden output for sunset.avif.
gen/testdata_exiftool/images/bep/sunset.avif.config.json	Adds golden {width,height} for sunset.avif.
gen/testdata_exiftool/images/bep/sunrise.avif.json	Adds EXIFTool golden output for sunrise.avif.
gen/testdata_exiftool/images/bep/sunrise.avif.config.json	Adds golden {width,height} for sunrise.avif.

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