Add multivariant output support to Membrane.Transcoder

[khamilowicz]

Summary

• On-request output pads — Membrane.Transcoder now exposes availability: :on_request output pads, each accepting independent output_stream_format, transcoding_policy, and native_acceleration options via via_out. Single-output backward-compatible usage is unchanged.
• Fan-out via Tee — multi-output mode inserts a Membrane.Tee.Parallel between the connector and per-output transcoding chains, created atomically with all outputs in one spec to avoid the race where the Tee could receive data before any output is connected.
• Example — examples/multivariant_output.exs demonstrates a single H264 input transcoded simultaneously to H264, H265, and VP8 outputs.
• Tests — 5 new integration tests cover multivariant video and audio scenarios. Content correctness is verified by asserting each multivariant output is byte-identical to the equivalent single-output pipeline run.

Usage

# Multiple outputs
child(:transcoder, Membrane.Transcoder),
get_child(:transcoder)
|> via_out(Pad.ref(:output, 0), options: [output_stream_format: H264])
|> child(:hd_sink, Membrane.File.Sink),
get_child(:transcoder)
|> via_out(Pad.ref(:output, 1), options: [output_stream_format: VP8])
|> child(:vp8_sink, Membrane.File.Sink)

Test plan

• mix test --exclude vk — 54 tests, 0 failures
• Single-output backward-compatible usage unchanged (existing test suite covers this)
• Multivariant output produces byte-identical results to equivalent single-output pipelines
• Run elixir examples/multivariant_output.exs to verify the example produces three non-empty output files

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