`struct Rav1dFrameContext_frame_thread`: backport memory optimization from dav1d 1.3.0 #3836

Job	Run time
test-on-macos-latest	5m 9s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	2m 7s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	2m 21s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	2m 3s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2)	2m 54s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 1)	2m 34s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 2)	3m 23s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	2m 25s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	2m 27s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	2m 2s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2)	2m 3s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 1)	2m 16s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 2)	2m 40s
	34m 24s

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