Correctly use as_hsla_f32 in Add<Color> and AddAssign<Color>, fixes #5543

[vertesians]

Probably a copy-paste error, but Add<Color> and AddAssign<Color> should use rhs.as_hlsa_f32() instead of rhs.as_linear_rgba_f32() when the LHS is a Color::Hsla. Fixes #5543.