Add `#[must_use]` attribute to some `DynamicImage` methods #1453

Job	Run time
build_benchmarks	50s
Fuzz targets (cargo-fuzz)	51s
Fuzz targets (afl)	2m 7s
cargo-deny	19s
cargo-semver-checks	38s
clippy	2m 2s
public_private_dependencies	23s
rustfmt	11s
test_avif_decoding	1m 21s
test_other_archs (powerpc-unknown-linux-gnu)	2m 36s
test_features	37s
test_other_archs (i686-unknown-linux-gnu)	1m 42s
test_toolchains (1.67.1)	30s
test_features (default)	1m 55s
test_toolchains (nightly)	1m 44s
test_features (rayon)	47s
test_toolchains (beta)	1m 40s
test_features (avif)	1m 11s
test_features (bmp)	40s
test_features (dds)	36s
test_features (exr)	47s
test_features (ff)	32s
test_features (gif)	42s
test_features (hdr)	47s
test_features (ico)	46s
test_features (jpeg)	38s
test_features (png)	38s
test_features (pnm)	32s
test_features (qoi)	37s
test_features (tga)	25s
test_features (tiff)	51s
test_features (webp)	39s
	30m 34s

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