Headless decode primitive, the mirror of the encoder: VaapiDecoder opens a file,
hardware-decodes H.264 into VAAPI NV12 surfaces (hw_device_ctx + a get_format
callback selecting AV_PIX_FMT_VAAPI), maps each surface to a DRM-PRIME DMA-BUF,
and imports it as two wgpu plane textures via the existing dmabuf::import_raw —
the same path the encoder uses, in the read direction. Frames stay GPU-resident
(no CPU copy).
Validated by a round-trip test: encode solid gray with ZeroCopyEncoder, decode
it back, read the Y plane (mean ≈ 128). All 9 crate tests pass on the container's
Intel GPU.
Next (Stage 3): wire this into VideoManager/get_frame so the compositor consumes
a GPU texture directly (no write_texture upload), with software decode fallback.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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