Add missing map2/apply2 functions to `Pixel` trait #1402

Job	Run time
test_other_archs (powerpc-unknown-linux-gnu)	2m 42s
test_features	29s
test_other_archs (i686-unknown-linux-gnu)	1m 41s
test_avif_decoding	1m 35s
test_features (default)	1m 43s
test_toolchains (1.67.1)	30s
test_features (rayon)	30s
test_toolchains (nightly)	1m 6s
test_features (avif)	1m 3s
Fuzz targets (afl)	2m 5s
test_toolchains (beta)	1m 48s
test_features (bmp)	47s
build_benchmarks	46s
test_features (dds)	31s
test_features (exr)	44s
Fuzz targets (cargo-fuzz)	52s
cargo-deny	19s
test_features (ff)	40s
test_features (gif)	37s
clippy	1m 58s
test_features (hdr)	27s
public_private_dependencies	23s
rustfmt	7s
test_features (ico)	45s
test_features (jpeg)	29s
test_features (png)	34s
test_features (pnm)	35s
test_features (qoi)	29s
test_features (tga)	36s
test_features (tiff)	49s
test_features (webp)	33s
	28m 13s

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