Use `chunks_exact_mut` in PNG decoder #1167

Job	Run time
Fuzz targets (afl)	1m 43s
Fuzz targets (cargo-fuzz)	4m 22s
cargo-deny	20s
build_benchmarks	43s
clippy	2m 8s
public_private_dependencies	17s
rustfmt	4s
test_avif	1m 27s
test_avif_decoding	1m 51s
test_features	35s
test_features (default)	1m 19s
test_other_archs (powerpc-unknown-linux-gnu, default)	3m 23s
test_toolchains (1.61.0)	33s
test_features (gif)	53s
test_other_archs (powerpc-unknown-linux-gnu, webp-encoder)	2m 10s
verify_msrv	2m 15s
test_toolchains (nightly)	2m 10s
test_other_archs (i686-unknown-linux-gnu, default)	1m 25s
test_features (jpeg)	38s
test_other_archs (i686-unknown-linux-gnu, webp-encoder)	54s
test_toolchains (beta)	1m 37s
test_features (png)	37s
test_features (tiff)	28s
test_features (ico)	36s
test_features (pnm)	24s
test_features (tga)	38s
test_features (webp)	37s
test_features (bmp)	34s
test_features (hdr)	29s
test_features (dxt)	25s
test_features (dds)	22s
test_features (farbfeld)	32s
test_features (openexr)	47s
test_features (jpeg_rayon)	31s
test_features (webp-encoder)	47s
	38m 34s

Provide feedback