h264: fix DTS extraction in case of long NALUs

[hsnks100]

The minimum bytes to estimate getPictureOrderCount is not 6.

A failure may occur when reading picOrderCntLsb due to bits truncated by EmulationPreventionRemove.

My idea is that you shouldn't cut it with [:6], you should pass the whole thing.

[hsnks100]

In addition, I think that emulation prevention processing should be done after receiving the packet.

[hsnks100]

@aler9 review my code.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (1b36c5c) 84.21% compared to head (7cf968f) 84.16%.

Files	Patch %	Lines
pkg/codecs/h264/dts_extractor.go	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   84.21%   84.16%   -0.06%     
==========================================
  Files          71       71              
  Lines        5532     5533       +1     
==========================================
- Hits         4659     4657       -2     
- Misses        650      652       +2     
- Partials      223      224       +1     

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[aler9]

Hello, the problem of this PR is that it strongly decreases performance, since NALUS can be really long, and if the truncation is removed, the entire NALU is scanned and copied in another buffer.

Just edit the truncation to the max possible extent needed to extract the POC, that should be

(max_size(first_mb_in_slice) + max_size(slice_type) + max_size(pic_parameter_set_id) +
max_size(frame_num) + max_size(pic_order_cnt_lsb)) * 4 / 3 =
(3 * max_size(golomb) + (max(Log2MaxFrameNumMinus4) + 4) / 8 + (max(Log2MaxPicOrderCntLsbMinus4) + 4) / 8) * 4 / 3 =
(3 * 4 + 2 + 2) * 4 / 3 = 22

[hsnks100]

I applied review.

[aler9]

i improved performance a little and merged, thanks.