Implement DoubleEndedIterator on `enumerate_*()` pixel iterators #1538

Job	Run time
cargo-deny	20s
build_benchmarks	45s
Fuzz targets (afl)	2m 14s
Fuzz targets (cargo-fuzz)	57s
cargo-semver-checks	22s
clippy	2m 7s
public_private_dependencies	23s
rustfmt	10s
test_avif_decoding	1m 41s
test_features	56s
test_other_archs (powerpc-unknown-linux-gnu)	2m 39s
test_features (default)	1m 59s
test_toolchains (1.67.1)	1m 5s
test_other_archs (i686-unknown-linux-gnu)	1m 56s
test_toolchains (nightly)	1m 48s
test_features (rayon)	39s
test_features (avif)	1m 38s
test_toolchains (beta)	1m 55s
test_features (bmp)	53s
test_features (dds)	25s
test_features (exr)	40s
test_features (ff)	28s
test_features (gif)	1m 17s
test_features (hdr)	1m 19s
test_features (ico)	49s
test_features (jpeg)	57s
test_features (png)	23s
test_features (pnm)	1m 6s
test_features (qoi)	17s
test_features (tga)	1m 15s
test_features (tiff)	35s
test_features (webp)	51s
	34m 49s

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