Make `with_alpha_factor` take the `Brush` by value #115

Job	Run time
cargo check (msrv) (wasm32)	27s
cargo clippy (wasm32)	32s
cargo check (msrv) (windows-latest)	1m 26s
cargo clippy (windows-latest)	1m 11s
cargo check (msrv) (macos-latest)	46s
cargo clippy (macos-latest)	43s
cargo doc	19s
cargo check (msrv) (ubuntu-latest)	44s
cargo fmt	23s
cargo clippy (ubuntu-latest)	38s
cargo test (windows-latest)	57s
cargo test (wasm32)	25s
typos	3s
cargo test (macos-latest)	28s
cargo test (ubuntu-latest)	27s
	9m 29s

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