reduce low-level byte fiddling in openexr encoder (#2012) #1011

Job	Run time
Fuzz targets (cargo-fuzz)	6m 49s
build_benchmarks	1m 35s
Fuzz targets (afl)	1m 57s
cargo-deny	31s
clippy	4m 53s
public_private_dependencies	25s
rustfmt	17s
test_avif	1m 51s
test_avif_decoding	1m 51s
test_big_endian	1m 20s
test_big_endian (default)	3m 26s
test_toolchains (1.61.0)	8m 7s
test_features	58s
test_big_endian (webp)	1m 50s
test_toolchains (nightly)	2m 56s
verify_msrv	4m 28s
test_features (default)	1m 57s
test_toolchains (beta)	2m 9s
test_big_endian (webp-encoder)	3m 3s
test_features (gif)	53s
test_features (jpeg)	1m 0s
test_features (png)	1m 0s
test_features (tiff)	1m 14s
test_features (ico)	1m 5s
test_features (pnm)	59s
test_features (tga)	1m 9s
test_features (webp)	1m 14s
test_features (bmp)	59s
test_features (hdr)	56s
test_features (dxt)	58s
test_features (dds)	56s
test_features (farbfeld)	51s
test_features (openexr)	1m 26s
test_features (jpeg_rayon)	1m 4s
test_features (webp-encoder)	1m 34s
	1h 7m 41s

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