Enable AV1 support

[wsippel]

Add AV1 support to the client and enable AV1 VA-API encoding on Linux. Tested on a Radeon 7900XTX GPU and Quest 3.

[zmerp]

You need to rebase this PR and fix the CI checks

[wsippel]

I hope I rebased this correctly. I also don't get the failed check. Error: Unable to create clippy annotations! Reason: HttpError: Resource not accessible by integration looks like a problem with the CI itself, no? And the three warnings the check spits out are all in files this PR doesn't touch.

[zmerp]

Uhm I think you didn't rebase correctly. The amount of changes doesn't correspond to the actual changes. To see the actual error in CI you need to expand the section.

[wsippel]

How can that even happen? I just followed a guide I found online and did:

git remote add upstream https://github.com/alvr-org/ALVR.git
git fetch upstream
git rebase upstream/master
git push origin master --force

[zmerp]

Oh sorry, nevermind. the large change is for the av1 backport patch for ffmpeg, I missed that at first. so LGTM :)