Add `#[must_use]` attribute to some `DynamicImage` methods #1460

Job	Run time
test_other_archs (powerpc-unknown-linux-gnu)	2m 30s
test_toolchains (1.67.1)	42s
test_other_archs (i686-unknown-linux-gnu)	1m 31s
Fuzz targets (afl)	2m 3s
build_benchmarks	49s
Fuzz targets (cargo-fuzz)	50s
test_toolchains (nightly)	1m 2s
test_toolchains (beta)	1m 51s
cargo-deny	18s
cargo-semver-checks	28s
clippy	2m 6s
public_private_dependencies	25s
rustfmt	9s
test_avif_decoding	1m 24s
test_features	34s
test_features (default)	1m 49s
test_features (rayon)	34s
test_features (avif)	59s
test_features (bmp)	39s
test_features (dds)	25s
test_features (exr)	41s
test_features (ff)	28s
test_features (gif)	29s
test_features (hdr)	46s
test_features (ico)	41s
test_features (jpeg)	42s
test_features (png)	39s
test_features (pnm)	27s
test_features (qoi)	28s
test_features (tga)	40s
test_features (tiff)	33s
test_features (webp)	32s
	28m 14s

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