Improve encode_gray performance in BmpEncoder #1364

Job	Run time	Billable time
cargo-deny	22s	0s
Fuzz targets (cargo-fuzz)	50s	0s
build_benchmarks	52s	0s
Fuzz targets (afl)	2m 15s	0s
clippy	2m 21s	0s
public_private_dependencies	28s	0s
rustfmt	15s	0s
test_avif_decoding	1m 15s	0s
test_other_archs (powerpc-unknown-linux-gnu)	2m 21s	0s
test_toolchains (1.67.1)	32s	0s
test_features	34s	0s
test_toolchains (nightly)	2m 0s	0s
test_other_archs (i686-unknown-linux-gnu)	1m 28s	0s
test_features (default)	1m 49s	0s
test_toolchains (beta)	1m 24s	0s
test_features (rayon)	50s	0s
test_features (avif)	1m 11s	0s
test_features (bmp)	34s	0s
test_features (dds)	27s	0s
test_features (exr)	48s	0s
test_features (ff)	41s	0s
test_features (gif)	30s	0s
test_features (hdr)	39s	0s
test_features (ico)	44s	0s
test_features (jpeg)	35s	0s
test_features (png)	33s	0s
test_features (pnm)	40s	0s
test_features (qoi)	28s	0s
test_features (tga)	34s	0s
test_features (tiff)	30s	0s
test_features (webp)	38s	0s
	29m 8s	0s

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