Conrod: Re-factor the `color` module

[mitchmindtree]

The current enum Color type is copied from the days where Conrod used elmesque (a functional, elm-inspired rust graphics library). These days it really does not make sense for Color to be an enum.

I think we should change the Color, Rgba and Hsla types to this:

pub struct Rgba(pub [f32; 4]);
pub struct Hsla(pub [f32; 4]);
pub type Color = Rgba;

[LaylConway]

If Rgba is actually sRGB format, perhaps it's clearer if it's named Srgba.

Edit: If not, the API docs should include that it's Linear

[Yam76]

It seems like Rgba and Hsla structs are in the code. Is there anything else to be done?

Also, why have a type alias for Color? Why not just force users to choose either Rgba or Hsla, since they are both already in the color module in the first place?

[mitchmindtree]

Just an update of where I'm at on this - If I were to revisit this today, I'd likely remove hsl support, only provide a minimal Srgba type and then refer users to the palette crate if they would like to work with other colour spaces.