Let the Demuxer read straight into the packet it hands out

[NicolasHug]

next_packet() has to return a packet the caller owns indefinitely: the
Python-level Packet can be held, moved to another thread, and decoded much
later. That is a UniqueAVPacket, and the AutoAVPacket / ReferenceAVPacket
pair is for the opposite thing - a loop that reuses one allocation and
unrefs at the end of each iteration.

So next_packet() was using the loop idiom and then undoing it: it read into
the shared packet, allocated a second one anyway, and moved the reference
across. Neither half of the idiom paid for itself - the allocation isn't
saved, and av_packet_move_ref() leaves the source blank so the scope-exit
unref has nothing to do.

read_next_packet() has no opinion on how the packet is owned, it just needs
somewhere to write, so it takes the AVPacket itself. Its callers keep the
ownership type that suits them: a ReferenceAVPacket for the two read loops
(SingleStreamDecoder and scan), a UniqueAVPacket for next_packet(). It also
unrefs between iterations, which av_read_frame() documents as the caller's
job rather than something it does for you.

---

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✅ You can merge normally! (4 Unrelated Failures)

As of commit d0618ef with merge base 6f705bb ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Linux CUDA x86 / install-and-test (3.10, 13.0, 6) (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Linux CUDA x86 / install-and-test (3.10, 13.0, 4) (gh) (trunk failure)
• Linux CUDA x86 / install-and-test (3.10, 13.2, 4) (gh) (trunk failure)
  test/test_decoders.py::TestBlocks::test_cpu_fallback_upload_keeps_full_range[422]
• Linux CUDA x86 / install-and-test (3.10, 13.2, 6) (gh) (trunk failure)

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