reduce low-level byte fiddling in openexr encoder #991

Job	Run time
test_features	43s
test_features (default)	1m 52s
Fuzz targets (cargo-fuzz)	7m 4s
test_features (gif)	54s
clippy	3m 18s
test_features (jpeg)	1m 5s
Fuzz targets (afl)	2m 15s
test_features (png)	56s
test_avif	2m 40s
test_avif_decoding	1m 57s
test_features (tiff)	1m 2s
test_big_endian	1m 32s
test_features (ico)	58s
test_big_endian (default)	3m 24s
test_toolchains (1.61.0)	3m 34s
test_features (pnm)	1m 4s
test_big_endian (webp)	2m 17s
test_toolchains (nightly)	1m 50s
test_features (tga)	48s
test_big_endian (webp-encoder)	3m 7s
test_toolchains (beta)	2m 17s
test_features (webp)	53s
test_features (bmp)	49s
build_benchmarks	1m 7s
test_features (hdr)	51s
cargo-deny	29s
test_features (dxt)	56s
test_features (dds)	1m 4s
public_private_dependencies	25s
test_features (farbfeld)	50s
rustfmt	18s
test_features (openexr)	1m 12s
test_features (jpeg_rayon)	1m 19s
verify_msrv	3m 27s
test_features (webp-encoder)	1m 29s
	59m 46s

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