Use ExtendedColorType when encoding #1258

Job	Run time
test_avif_decoding	1m 30s
test_features	15s
test_features (default)	2m 3s
test_features (rayon)	24s
Fuzz targets (afl)	2m 5s
Fuzz targets (cargo-fuzz)	6m 31s
test_features (avif)	1m 4s
build_benchmarks	51s
test_features (bmp)	28s
test_features (dds)	18s
clippy	1m 29s
public_private_dependencies	25s
test_features (exr)	41s
test_other_archs (powerpc-unknown-linux-gnu)	3m 54s
test_features (ff)	21s
test_other_archs (i686-unknown-linux-gnu)	1m 56s
test_toolchains (1.67.1)	47s
test_features (gif)	43s
test_toolchains (nightly)	1m 41s
test_features (hdr)	28s
test_toolchains (beta)	1m 43s
cargo-deny	23s
test_features (ico)	26s
test_features (jpeg)	26s
test_features (png)	33s
rustfmt	9s
test_features (pnm)	21s
test_features (qoi)	20s
test_features (tga)	26s
test_features (tiff)	44s
test_features (webp)	29s
	33m 54s

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