Fix a race condition in VAAPI CSC to YUV420P #1133

Job	Run time
build_debian / Build FFmpeg (bullseye, amd64)	55m 10s
build_ubuntu / Build FFmpeg (focal, amd64)	1h 3m 35s
build_debian / Build FFmpeg (bullseye, arm64)	14m 27s
build_ubuntu / Build FFmpeg (focal, arm64)	15m 49s
build_portable_linux / Build Portable FFmpeg (amd64)	46m 17s
build_debian / Build FFmpeg (bullseye, armhf)	13m 52s
build_portable_mac / Build Portable FFmpeg (macos-13, x86_64)	1h 0m 2s
build_ubuntu / Build FFmpeg (focal, armhf)	14m 24s
build_portable_linux / Build Portable FFmpeg (arm64)	46m 0s
build_debian / Build FFmpeg (bookworm, amd64)	1h 4m 22s
build_portable_mac / Build Portable FFmpeg (macos-14, arm64)	19m 53s
build_ubuntu / Build FFmpeg (jammy, amd64)	55m 46s
build_debian / Build FFmpeg (bookworm, arm64)	14m 29s
build_ubuntu / Build FFmpeg (jammy, arm64)	15m 26s
build_debian / Build FFmpeg (bookworm, armhf)	13m 15s
build_portable_windows_clang / Build Portable FFmpeg (CLANG64, win64)	1h 6m 23s
build_ubuntu / Build FFmpeg (jammy, armhf)	14m 39s
build_ubuntu / Build FFmpeg (noble, amd64)	59m 54s
build_portable_windows / Build Portable FFmpeg (win64)	37m 0s
build_ubuntu / Build FFmpeg (noble, arm64)	16m 29s
build_ubuntu / Build FFmpeg (noble, armhf)	14m 29s
build_portable_windows / Publish Portable Release	0s
build_portable_linux / Publish Portable Release	0s
build_portable_mac / Publish Portable Release	0s
build_ubuntu / Publish Release	0s
build_debian / Publish Release	0s
build_portable_windows_clang / Publish Portable Release	0s
	12h 1m 41s

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