See what the added code comment says. Normally when this is needed, it's
the cover art case. But this flag is not set when using an external
image. This gives weird seek behavior, because the frame will be
"normally" displayed for its determined duration, and during normal
video playback, the video pts will be used - which is always 0 here.
This should happen only if audio is active. Otherwise, we're more or
less in image viewer mode, where the image should be displayed for a
configured duration.