src/vs/base/common/color.ts/Color/_flatten
class Color:	private static _flatten(foreground: Color, background: Color) {
		const backgroundAlpha = 1 - foreground.rgba.a;
		return new Color(new RGBA(
			backgroundAlpha * background.rgba.r + foreground.rgba.a * foreground.rgba.r,
			backgroundAlpha * background.rgba.g + foreground.rgba.a * foreground.rgba.g,
			backgroundAlpha * background.rgba.b + foreground.rgba.a * foreground.rgba.b
		));
	}