Port of stb_image optimized paeth unfiltering #631

Job	Run time
powerpc_cross	9s
rustfmt	11s
build (1.57.0, ubuntu-latest)	55s
feature_check (ubuntu-latest)	1m 0s
build (1.57.0, windows-latest)	1m 19s
feature_check (macos-latest)	35s
build (1.57.0, macos-latest)	1m 6s
feature_check (unstable, ubuntu-latest)	58s
test_all (ubuntu-latest)	3m 5s
build (stable, ubuntu-latest)	14s
feature_check (unstable, macos-latest)	44s
test_all (macos-latest)	2m 26s
feature_check (benchmarks, ubuntu-latest)	1m 1s
build (stable, windows-latest)	43s
feature_check (benchmarks, macos-latest)	43s
build (stable, macos-latest)	16s
build (beta, ubuntu-latest)	16s
build (beta, windows-latest)	31s
build (beta, macos-latest)	17s
build (nightly, ubuntu-latest)	13s
build (nightly, windows-latest)	27s
build (nightly, macos-latest)	22s
	17m 31s

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