Add `with_alpha_factor` to `BrushRef`/`Brush` #108

Job	Run time
cargo test (windows-latest)	55s
cargo test (macos-latest)	18s
cargo test (ubuntu-latest)	11s
cargo clippy (windows-latest)	16s
cargo check (msrv) (windows-latest)	16s
cargo check (msrv) (wasm32)	18s
cargo clippy (macos-latest)	21s
cargo check (msrv) (macos-latest)	20s
cargo clippy (wasm32)	35s
cargo clippy (ubuntu-latest)	15s
cargo check (msrv) (ubuntu-latest)	14s
cargo doc	34s
cargo fmt	24s
cargo test (wasm32)	17s
typos	8s
	5m 22s

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