Change Color to be a struct rather than an enum

[SpaceManiac]

This reduces the size of the type and eliminates the need to branch to read from it, increasing its usability, and bringing it closer to the conceptual Color type used by most libraries and SDL itself.

This is a breaking change, but the RGB and RGBA constructor functions should make the impact minimal for anybody not manually matching.

I suppose I should make a note about some of the other inconsistencies; raw() is private, "because reasons", but Color implements Into<ll::SDL_Color> meaning the conversion is publicly available as safe anyways, and the ttf module seems to independently re-implement this function for no discernible reason. This could perhaps also be improved.

[Cobrand]

This is definitely a breaking change but I think it is for the greater good as well. I do match manually these enums in some of my projects, and having to rewrite this will be both a pain and a relief for later.

This is a pretty huge breaking change though; I've added a changelog file, would you mind adding a short description of your changes and maybe linking to this issue, guiding people on how to migrate to the new Color struct ? It wouldn't be fair to add such a breaking change a not notify end users IMO

[SpaceManiac]

Added some information to the changelog with new usage examples. Also a little bit tidied raw() and its usage in ttf.

[Cobrand]

Thanks !