[Meta] Playback bugs when changing speed

mpv has a variety of subtle issues with when playback speed is changed, including A/V desync, seeking-like behavior, audio and/or video stuttering, etc.

Apparently, this may be a bit complicated to fix in a way that solves all of the issues simultaneously without other side effects, which will take some time, so I am creating this meta issue to track further progress here

Ideally, changing playback speed

• should be instantaneous and take effect immediately on both the video and audio streams
• should keep audio and video in sync at all times
• should not trigger a seek or cause seeking-like behavior
• should not introduce gaps or cause stuttering in video or audio
• should report correct values for time-pos and audio-pts at all times

See also

• https://github.com/mpv-player/mpv/issues/13513
• https://github.com/mpv-player/mpv/issues/13613
• https://github.com/mpv-player/mpv/issues/13622
• https://github.com/mpv-player/mpv/pull/13614

No source file, test, or entry point is identified, so start by reviewing related issues #13513, #13613, and #13622, along with pull request #13614. Done would require playback-speed changes to apply immediately without desynchronization, seeking-like behavior, stuttering, gaps, or incorrect time-pos and audio-pts values.