`struct Rav1dFrameContext_lf::cdef_line`: Convert pointers to offsets #3563

Job	Run time
test-on-macos-latest	4m 44s
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, release, --release --no-default-features --featu...	2m 4s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	2m 0s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2)	2m 26s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 1)	2m 37s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 2)	3m 35s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	2m 9s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	2m 24s
test-on-ubuntu-latest (i686-unknown-linux-gnu, release, --release --no-default-features --feature...	1m 54s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2)	2m 14s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 1)	1m 57s
test-on-ubuntu-latest (i686-unknown-linux-gnu, opt-dev, --profile opt-dev, 2, -f 2)	2m 45s
	32m 52s

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