Fix VP9 decoding on iOS

[aler9]

Description

The current implementation of the VP9 payloader produces payloads that are not compatible with iOS. This is because the payloader provides only the muxing strategy called "flexible mode".

According to the VP9 RFC draft, there are two ways to wrap VP9 frames into RTP packets: the "flexible mode" and the "non-flexible mode", with the latter being the preferred one for live-streaming applications. In particular, all browsers encodes VP9 RTP packets in the "non-flexible mode", while iOS supports decoding RTP packets in this mode only, and this is probably a problem shared by other implementations.

This patch improves the VP9 payloader by adding support for the "non-flexible mode". The "flexible mode" is retained and a flag is provided to perform the switch between the two modes.

Reference issue

bluenviron/mediamtx#3060

[codecov]

Codecov Report

Attention: Patch coverage is 67.27273% with 72 lines in your changes are missing coverage. Please review.

Project coverage is 84.10%. Comparing base (12646b6) to head (251d31c).

Files	Patch %	Lines
codecs/vp9/header.go	40.90%	47 Missing and 18 partials ⚠️
codecs/vp9/bits.go	88.57%	2 Missing and 2 partials ⚠️
codecs/vp9_packet.go	96.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
- Coverage   86.22%   84.10%   -2.12%     
==========================================
  Files          22       24       +2     
  Lines        1749     1950     +201     
==========================================
+ Hits         1508     1640     +132     
- Misses        204      253      +49     
- Partials       37       57      +20     

Flag	Coverage Δ
go	84.10% <67.27%> (-2.12%)	⬇️
wasm	83.48% <65.45%> (-2.28%)	⬇️

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[Sean-Der]

Thank you @aler9! I am in full support of this, happy to review w/e you are ready.

If Flexible mode doesn't have value (and is a burden) we could drop it.

[aler9]

Hello @Sean-Der, now everything is ready for review, thanks for being available.
Regarding flexible mode, i don't know whether it has a value or not, keeping it is not a burden and avoids breakages in existing implementations.

[Sean-Der]

LGTM @aler9! Thank you so much :)