Store a UniqueAVFrame in the blocks frame handle

[NicolasHug]

The frame handle held a raw AVFrame*, so _blocks_convert_frame() had to
build a UniqueAVFrame around a pointer the handle already owned and then
release() it again to undo the fabricated second owner. It only worked
because nothing downstream acted on that ownership.

Hold a UniqueAVFrame instead. It's an ordinary C++ object whose destructor
calls av_frame_free, so the generic wrap_pointer_to_tensor handle used
by the other blocks types applies and the bespoke AVFrame wrapper goes
away. _blocks_convert_frame() then has a UniqueAVFrame to pass by
reference, with no temporary owner in sight.

Costs one small allocation per frame for the wrapper object.

---

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[NicolasHug]

Superseded by #1598