cleanup of `fill_window` #1156

Job	Run time
Clippy	19s
QEMU build & test (stable, ubuntu-latest, s390x-unknown-linux-gnu, s390x-linux-gnu-gcc)	41s
Clippy (--no-default-features --features="c-allocator")	22s
QEMU build & test (stable, ubuntu-latest, i686-unknown-linux-gnu, i686-linux-gnu-gcc)	42s
cargo-c dynamic library	9s
Clippy (--no-default-features --features="rust-allocator")	16s
Build and test (stable, ubuntu-latest, x86_64-unknown-linux-gnu)	53s
Build and test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	59s
Smoke-test fuzzing targets (stable, macos-14, aarch64-apple-darwin)	2m 33s
Build and test (beta, ubuntu-latest, x86_64-unknown-linux-gnu)	1m 0s
Smoke-test fuzzing targets (default)	2m 30s
vanilla dynamic library	12s
Build and test (stable, macos-latest, x86_64-apple-darwin)	1m 6s
Smoke-test fuzzing targets (c-allocator)	2m 32s
Smoke-test fuzzing targets (rust-allocator)	2m 32s
wasm32	38s
Build and test (stable, macos-14, aarch64-apple-darwin)	55s
Miri	1m 40s
run flate2 test suite	17s
Build and test (stable-x86_64-gnu, windows-2022, x86_64-pc-windows-gnu)	2m 12s
	22m 28s

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