obs-ffmpeg: Disable NVENC/AMF DTS adjustment for AV1

[palana]

Description

Aligns AV1 DTS and PTS, since AV1 doesn't use b-frame delay (similar to VP9/VP8). AV1 instead handles reordering of frames transparently by outputting frames that aren't displayed in the same encode call; e.g. with b-frames enabled the first time data is returned by these encoders there will generally be a keyframe in the output that can be displayed immediately, and the next time data is returned (with the next dts) will contain various frames that aren't displayed and one frame that is displayed

in the image below, frames with green underline are output in the same encode call:

consider also the ertmp (v1 and later) spec https://github.com/veovera/enhanced-rtmp/blob/6445c80106c418ebe6bb18602e1c86645b2c1778/docs/enhanced/enhanced-rtmp-v1.md where PacketTypeCodedFrames for AV1 does not include SI24 = [CompositionTime Offset]

Motivation and Context

AV1 timestamps are currently misaligned for NVENC/AMF in multi codec ladders for Enhanced Broadcasting/Multitrack Video on Twitch

How Has This Been Tested?

Twitch Enhanced Broadcasting Beta, inspected timestamps via broadcast performance metrics logs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[derrod]

I've been wondering about whether or not we need to do this since working on #10436 and learning about show/no-show frames, but didn't have time to look into it. Seems fine to me.

[RytoEX]

Is this something critical? While the NVENC code changed here was added in 30.2 (here 2f55f04), the AMF code is nearly two years old (2e5f900).

[derrod]

Is this something critical? While the NVENC code changed here was added in 30.2 (here 2f55f04), the AMF code is nearly two years old (2e5f900).

The NVENC code was only moved in 30.2, it has existed like that previously.

Given that this doesn't cause issues with recordings or streaming to YouTube, and Twitch has no plans of allowing AV1 all that soon anyway I do not think this is critical, but good to have for 31.0.