jpeg: add encodingProcess and numColorComponents SOF members

[enen92]

This adds support for extracting and storing some of the metadata that can be obtained from start of frame (SOF) tags in Jpeg files, namely:

• The encoding process
• The number of color components

Afaik only exiftool supports such "tags" see (SOF tags in https://exiftool.org/TagNames/JPEG.html#SOF). As they don't really belong to a metadata class like exif or iptc and are only specific of a specific format (jpeg) I added them as members of Exiv2:: JpegImage. Added a small sample and test to validate the implementation.

Motivation:
Kodi currently implements its own jpeg parser and can thus obtain these two properties (https://github.com/xbmc/xbmc/blob/4532c7441510a9c9ebfa2023dd129ce13544890b/xbmc/pictures/PictureInfoTag.cpp#L416-L441 https://github.com/xbmc/xbmc/blob/master/xbmc/pictures/JpegParse.cpp#L73-L87). With my aim to rewrite the implementation based on exiv2 (xbmc/xbmc#24109) they'll be removed if no alternative exists. So I decided to have a go at it and leave it to your consideration to collect feedback (and check if it's possible to have it upstream).

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Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (349c1f2) 63.89% compared to head (b9c167e) 63.90%.

Files	Patch %	Lines
src/jpgimage.cpp	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2874   +/-   ##
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  Coverage   63.89%   63.90%           
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  Files         103      104    +1     
  Lines       22381    22389    +8     
  Branches    10872    10876    +4     
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+ Hits        14301    14308    +7     
  Misses       5857     5857           
- Partials     2223     2224    +1     

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[enen92]

@neheb thanks for merging. May I ask if you guys have any rough time estimate for a future release/tag of the library? Just to have an idea of when it's best to have it merged into upstream Kodi.

[neheb]

@Mergifyio backport 0.28.x

best to do that if you want this in a release. current master will not be released in a long time.

[mergify]

backport 0.28.x

✅ Backports have been created

• #2887 jpeg: add encodingProcess and numColorComponents SOF members (backport #2874) has been created for branch 0.28.x but encountered conflicts

[enen92]

Thanks, I'll handle the backport tomorrow. I think this is the only required one. For all other contributions (mostly cmake options) we can carry the patches in the codebase since they only affect platforms for which we build statically anyway (and we already fulfilled our "submit upstream first" patch carrying principle).
Cheers