libobs: Ensure audio offsets are positive

[derrod]

Description

Only set audio offset if current DTS is positive.

Motivation and Context

DTS/PTS can be negative for "priming" samples produced by AAC/Opus encoders, currently this would result in the DTS/PTS being corrected to 0, which causes A/V desync by making the silent priming samples part of the actual presentation.

By leaving the negative DTS/PTS the FFmpeg muxer (and #10608) can correctly set the encoder delay and ensure synced playback.

How Has This Been Tested?

Recorded a few videos of the Twitch sync test and validated that sync is now equivalent to PCM (there still seems to be one frame of delay somewhere in OBS).

(Note: #10690 was included here)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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.