Fix a race condition in VAAPI CSC to YUV420P #1129

Job	Run time
build_portable_windows / Build Portable FFmpeg (win64)	36m 25s
build_ubuntu / Build FFmpeg (focal, amd64)	41m 12s
build_ubuntu / Build FFmpeg (focal, arm64)	16m 11s
build_debian / Build FFmpeg (bullseye, amd64)	41m 14s
build_portable_linux / Build Portable FFmpeg (amd64)	41m 10s
build_ubuntu / Build FFmpeg (focal, armhf)	14m 13s
build_debian / Build FFmpeg (bullseye, arm64)	14m 30s
build_portable_mac / Build Portable FFmpeg (macos-13, x86_64)	31m 36s
build_portable_linux / Build Portable FFmpeg (arm64)	41m 15s
build_ubuntu / Build FFmpeg (jammy, amd64)	41m 11s
build_debian / Build FFmpeg (bullseye, armhf)	15m 39s
build_portable_mac / Build Portable FFmpeg (macos-14, arm64)	20m 49s
build_ubuntu / Build FFmpeg (jammy, arm64)	16m 13s
build_debian / Build FFmpeg (bookworm, amd64)	41m 12s
build_ubuntu / Build FFmpeg (jammy, armhf)	14m 32s
build_portable_windows_clang / Build Portable FFmpeg (CLANG64, win64)	25m 6s
build_debian / Build FFmpeg (bookworm, arm64)	15m 28s
build_ubuntu / Build FFmpeg (noble, amd64)	41m 9s
build_debian / Build FFmpeg (bookworm, armhf)	13m 18s
build_ubuntu / Build FFmpeg (noble, arm64)	16m 4s
build_ubuntu / Build FFmpeg (noble, armhf)	14m 20s
build_portable_windows_clang / Publish Portable Release	0s
build_portable_mac / Publish Portable Release	0s
build_portable_windows / Publish Portable Release	0s
build_debian / Publish Release	1s
build_ubuntu / Publish Release	1s
build_portable_linux / Publish Portable Release	1s
	9h 12m 50s

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