Small refactoring of paeth filter logic #639

Job	Run time
rustfmt	9s
powerpc_cross	7s
feature_check (ubuntu-latest)	1m 4s
build (1.57.0, ubuntu-latest)	1m 6s
feature_check (macos-latest)	41s
test_all (ubuntu-latest)	3m 2s
feature_check (unstable, ubuntu-latest)	1m 1s
build (1.57.0, windows-latest)	1m 25s
build (1.57.0, macos-latest)	1m 8s
test_all (macos-latest)	2m 23s
feature_check (unstable, macos-latest)	34s
build (stable, ubuntu-latest)	7s
feature_check (benchmarks, ubuntu-latest)	1m 0s
build (stable, windows-latest)	26s
feature_check (benchmarks, macos-latest)	48s
build (stable, macos-latest)	9s
build (beta, ubuntu-latest)	12s
build (beta, windows-latest)	28s
build (beta, macos-latest)	20s
build (nightly, ubuntu-latest)	13s
build (nightly, windows-latest)	36s
build (nightly, macos-latest)	18s
	17m 17s

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