Function rgb_to_hsl

fn rgb_to_hsl([r, g, b]: [f32; 3], hue_hack: bool) -> [f32; 3]

Convert RGB to HSL.

Reference: § 7.2 of CSS Color 4 spec.

See https://github.com/w3c/csswg-drafts/issues/10695 for an explanation of why hue_hack is needed.