Videos render as grey blobs for first few frames

Operating System Info

macOS 11.5

Other OS

No response

OBS Studio Version

27.0.1

OBS Studio Version (Other)

No response

OBS Studio Log URL

https://obsproject.com/logs/iMtIcCPaUcbCFDtI

OBS Studio Crash Log URL

No response

Expected Behavior

Videos should be rendered correctly from the first frame.

Current Behavior

Observed in portrait videos from phones (mov/mp4 containing h.264/AVC and h.265/HEVC), mostly in VLC Video Source. Unsure if it applies to other videos or Media Source.

Looks like it's somehow missing the first keyframe and starting instead from a blank grey frame, and then rendering frame diffs until the next keyframe at which point it sorts itself out.

Steps to Reproduce

1. Add VLC Video Source to a scene.
2. Select the attached video as source.
3. Observe incorrect rendering.

https://user-images.githubusercontent.com/18594548/132258725-4ef35599-fab2-44c3-8262-60d5914ea876.MOV

Anything else we should know?

This is possibly related to #5250 but after a glance at the code I think they're separate issues, hence the split.

VLC itself renders the videos correctly.

Start with the VLC Video Source path and reproduce the issue using the attached MOV on macOS 11.5, then inspect the provided OBS log. Compare the first rendered frames with VLC's output and verify the result across the stated H.264/HEVC cases; done means portrait videos render correctly from the first frame.