Build the full end-to-end zero-copy encoder, validated on Intel/VAAPI:
- render_nv12: fragment-shader RGBA->NV12 that renders luma/chroma into
the imported R8/RG8 plane render targets (compute storage can't write
the DMA-BUF-backed planes; render attachments can).
- dmabuf: import_raw imports an NV12 DMA-BUF by explicit layout; the two
plane images + shared memory are now destroyed by wgpu via texture_from_raw
drop callbacks (Arc MemoryGuard frees the memory once both images are
gone, in wgpu's wait-idle'd deferred pass) -- fixes the teardown segfault.
- encoder::ZeroCopyEncoder: renders an RGBA texture straight into a pooled
VAAPI surface (imports cached by VASurface id) and encodes with h264_vaapi.
encode_rgba + finish; the caller renders on device().
Tests: real-frame render into the surface matches the CPU NV12 reference,
and a 30-frame encode produces valid H.264 (ffprobe-verified) with clean
teardown. Not yet wired into the editor.
Skyler Lehmkuhl