vf_vapoursynth: fail gracefully if filter init requests frames

Some VS filters will requests frames from their parent filters while
they're initialized. Thy do this in a blocking manner, and
initialization will not succeed until the frame request is satisfied.
This deadlocked mpv, because we can feed frames to the filter only after
initialization is finished.

Return an error instead of deadlocking.

Note that we (probably) can handle frames being requested during init
fine, as long as the requests don't block initialization. But we can
distinguish this situation, and a simple test seems to indicate VS
usually doesn't do this.

See #1168.

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