When exporting H.264 with audio, try the gpu-video-encoder ZeroCopyEncoder:
render each frame to RGBA and hardware-encode it into a VAAPI surface
inline, on the encoder's own VAAPI-capable wgpu device — no GPU->CPU
readback, no swscale, no software-encoder thread. Falls back to the
existing software path verbatim when VAAPI/the device is unavailable
(non-Linux, non-H264, or init failure), so it's additive.
- VideoExportState gains zero_copy: Option<ZeroCopyVideo> (encoder + its own
vello renderer + ExportGpuResources + a reused RGBA target, all on the
encoder's device).
- start_video_with_audio_export builds it for H.264 and skips spawning the
software encoder thread when present.
- render_next_video_frame routes to a zero-copy arm that reuses
render_frame_to_gpu_rgba on the encoder's device, then encode_rgba; on the
last frame finish() writes the temp .mp4 and sets video_progress=Complete
so the existing mux runs. video_thread=None makes the mux join a no-op.
Separate export device (vs modifying the eframe device) keeps this contained
to export. Video-only export stays on the software path for now. Runtime
verification (an actual H.264 export) is pending — cannot run the editor in
the dev container.
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