h264: support SEI recovery point in DTS extractor

[keks2293]

Treat a SEI recovery point message (payload type 6) as a random access point, similarly to a CRA in H265, so the DTS extractor can (re)initialize its state from the first coded frame that follows the recovery point.

[codecov]

Codecov Report

❌ Patch coverage is 67.14286% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.79%. Comparing base (8f30c83) to head (cb397a8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/codecs/h264/dts_extractor.go	65.00%	12 Missing and 2 partials ⚠️
pkg/codecs/h264/sei.go	76.00%	4 Missing and 2 partials ⚠️
pkg/codecs/h264/is_random_access.go	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
- Coverage   86.06%   85.79%   -0.27%     
==========================================
  Files          98       99       +1     
  Lines        6129     6175      +46     
==========================================
+ Hits         5275     5298      +23     
- Misses        584      603      +19     
- Partials      270      274       +4     

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