Adds VAAPI DMA-BUF, D3D11VA shared-handle and VideoToolbox IOSurface
imports behind a tri-state outcome (imported / unsupported / failed),
planar textures with bounded residency and a CPU staging fallback, the
staged montage decode path, reference-counted decoder frames, VAAPI-first
device selection on Linux, and the host-GPU context plumbing used by the
app and worker. See docs/zh/plans/render-pipeline-threads.md (M5).
Mike-Solar
f188bc79e7