This tried to use ctx->pic (last decoded AVFrame) for the frame bounds.
However, if av_frame_unref() on the AVFrame is called, the function will
reset _all_ AVFrame fields, even those which are not involved with
memory management. As result, mpcodecs_config_vo() was called with 0
width/height, which made the function exit early, instead of
reconfiguring the filter chain.
Go back to using mpcodecs_config_vo() directly. (That's what it did
before this VDCTRL was originally introduced; the original reason for
it disappeared.)