Allow raster sources to disable alpha premultiplication

[Plantain]

Add a raster source option that controls whether raster tiles should be alpha-premultiplied during decode and texture upload.

Implementation of maplibre/maplibre-style-spec#1542

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (568dccd) to head (8f23c4c).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7235      +/-   ##
==========================================
- Coverage   92.71%   92.44%   -0.28%     
==========================================
  Files         289      289              
  Lines       24074    24079       +5     
  Branches     5112     5114       +2     
==========================================
- Hits        22320    22259      -61     
- Misses       1754     1820      +66     

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