add `with_alpha` method for `Color` to easily transform alpha in colors #1605

Job	Run time
cargo fmt	6s
Cross-compile (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 8s
Cross-compile (ubuntu-latest, x86_64-pc-windows-gnu)	1m 3s
Cross-compile (ubuntu-latest, wasm32-unknown-unknown)	1m 3s
Cross-compile (macos-latest, x86_64-apple-darwin)	52s
Cross-compile (windows-latest, x86_64-pc-windows-msvc)	1m 48s
	6m 0s

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