Lots of shit code for nothing. We probably could just use libavutil's
code for all of this. But for now go with this, since it tends to
prevent stupid terminal messages during probing (libavutil has no
mechanism to selectively suppress errors specifically during probing).
Ignores the "emulated" API flag (for avoiding vaapi/vdpau wrappers), but
it doesn't matter that much for -copy anyway.
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