Rewrite H265 depacketizer impl

[Juliapixel]

Description

Rewrites the mostly broken and inaccurate current implementation of H265 depacketization. The previous public APIs were marked as deprecated and replaced with the new XPacketizer and XDepacketizer namings.

The depacketizer implementation was tested to be able to receive an RTP stream from chrome and produce a correct file playable with ffplay, but the packetizer implementation did not produce playable video on chrome, so (if needed) it will be fixed and unprivated in a follow-up PR.

Based on RFC 7798

[Sean-Der]

This is amazing, thank you so much for doing this @Juliapixel :D

super excited to use this for Broadcast Box seems like most people have HW Accelerated H265 this is gonna help a lot.

[codecov]

Codecov Report

❌ Patch coverage is 69.37799% with 192 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.93%. Comparing base (5cd305b) to head (4f608ca).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
codecs/h265_packet.go	69.37%	131 Missing and 61 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
- Coverage   89.43%   81.93%   -7.50%     
==========================================
  Files          28       28              
  Lines        3077     3527     +450     
==========================================
+ Hits         2752     2890     +138     
- Misses        217      456     +239     
- Partials      108      181      +73     

Flag	Coverage Δ
go	81.93% <69.37%> (-7.50%)	⬇️

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[JoTurk]

Works great, thank you Julia.

[JoTurk]

H265Packet: old is comparable, new is not

Doesn't matter, and H265Packet is deprecated. Many of other codecs depacketizer aren't comparable neither.

The codecov diff is from removing the broken test and deprecating the old public helpers (most of them aren't usable).

Safe to merge.

[JoTurk]

merged in 1461347 (we had a race condition with both of us rebasing at the same time :)
Thank you.