support decoding M-JPEG streams with any quantization (#566)

[aler9]

Fixes #566

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 20 lines in your changes missing coverage. Please review.

Project coverage is 81.14%. Comparing base (13d994d) to head (978a577).
Report is 1 commits behind head on main.

Files	Patch %	Lines
pkg/format/rtpmjpeg/decoder.go	60.52%	9 Missing and 6 partials ⚠️
pkg/format/rtpmjpeg/header_jpeg.go	25.00%	2 Missing and 1 partial ⚠️
pkg/format/rtpmjpeg/header_quantization_table.go	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #583      +/-   ##
==========================================
+ Coverage   81.13%   81.14%   +0.01%     
==========================================
  Files         115      115              
  Lines       11554    11598      +44     
==========================================
+ Hits         9374     9411      +37     
- Misses       1773     1775       +2     
- Partials      407      412       +5     

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