client: fix computing absolute timestamp with multiple renditions

[aler9]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 5 lines in your changes missing coverage. Please review.

Project coverage is 72.93%. Comparing base (ed88408) to head (b36b12d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
client_time_conv_fmp4.go	85.71%	2 Missing and 1 partial ⚠️
client_stream_processor_fmp4.go	93.33%	0 Missing and 1 partial ⚠️
client_track.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   72.96%   72.93%   -0.04%     
==========================================
  Files          58       58              
  Lines        3951     3968      +17     
==========================================
+ Hits         2883     2894      +11     
- Misses        876      880       +4     
- Partials      192      194       +2     

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