heif: Deliver all XMP items, filter by content_type

[bep]

Fixes #67

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.55%. Comparing base (8e8cc43) to head (7335e9f).

Files with missing lines	Patch %	Lines
imagedecoder_heif.go	80.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   78.36%   76.55%   -1.82%     
==========================================
  Files          16       16              
  Lines        2108     2657     +549     
==========================================
+ Hits         1652     2034     +382     
- Misses        318      484     +166     
- Partials      138      139       +1     

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