Fix a race condition in VAAPI CSC to YUV420P #1135

Job	Run time
build_debian / Build FFmpeg (bullseye, amd64)	54m 16s
build_portable_linux / Build Portable FFmpeg (amd64)	1h 49m 59s
build_debian / Build FFmpeg (bullseye, arm64)	15m 0s
build_portable_linux / Build Portable FFmpeg (arm64)	1h 38m 44s
build_portable_mac / Build Portable FFmpeg (macos-13, x86_64)	44m 24s
build_debian / Build FFmpeg (bullseye, armhf)	13m 49s
build_portable_windows / Build Portable FFmpeg (win64)	35m 37s
build_portable_mac / Build Portable FFmpeg (macos-14, arm64)	20m 39s
build_debian / Build FFmpeg (bookworm, amd64)	1h 3m 34s
build_debian / Build FFmpeg (bookworm, arm64)	14m 36s
build_portable_windows_clang / Build Portable FFmpeg (CLANG64, win64)	1h 4m 32s
build_ubuntu / Build FFmpeg (focal, amd64)	1h 3m 34s
build_debian / Build FFmpeg (bookworm, armhf)	13m 24s
build_ubuntu / Build FFmpeg (focal, arm64)	15m 34s
build_ubuntu / Build FFmpeg (focal, armhf)	14m 14s
build_ubuntu / Build FFmpeg (jammy, amd64)	59m 19s
build_ubuntu / Build FFmpeg (jammy, arm64)	15m 48s
build_ubuntu / Build FFmpeg (jammy, armhf)	13m 48s
build_ubuntu / Build FFmpeg (noble, amd64)	59m 7s
build_ubuntu / Build FFmpeg (noble, arm64)	16m 22s
build_ubuntu / Build FFmpeg (noble, armhf)	14m 26s
build_portable_windows / Publish Portable Release	0s
build_portable_mac / Publish Portable Release	0s
build_debian / Publish Release	0s
build_ubuntu / Publish Release	0s
build_portable_windows_clang / Publish Portable Release	0s
build_portable_linux / Publish Portable Release	0s
	13h 40m 46s

Provide feedback